Nobody's sure whether to count the Worst Generation's win over the Emperors as an actual win. Kid and Law only knocked Big Mom off of Onigashima and removed her from the battle; they didn't really kill her and might not have even knocked her out. Likewise, not many believe that Kaidō was killed even after being driven into the ground and submerged in magma, nor do they believe this to be the case for Big Mom. People are more likely to attribute the volcanic eruption to their ultimate "defeat." In any case, it's hard to say whether the Worst Generation truly "beat" the Emperors in a straight fight.
However, it's not as if the Emperors are invincible. Excluding the fact that Luffy and co. are meant to inevitably surpass them on their paths to greatness, there are several instances of Emperors being "defeated" throughout One Piece. Looking back at some of the bigger "losses" might give a better idea of what a battle against an Emperor entails; what counts as a win, what counts as a loss, and whether certain conditions could have skewed the results. Once all of those definitions are squared away, it should be much easier to determine whether the Worst Generation truly "won" against the Emperors.
The ideal benchmark for challenging an Emperor was set during the War of Marineford. In this battle, Whitebeard, then the strongest man in the world, was subjected to all sorts of punishment before he died; he received over 460 injuries from swords, guns, and cannons; He also took a laser blast from Admiral Kizaru and a magma blast from Admiral Akainu that cost him half his head in the manga. Even after all of this, he kept on going before he finally died standing up.
There's an argument to be made for what might have happened if Whitebeard had been in better shape. Many fans argue that his age and his poor health were major contributing factors to his demise. Regardless, he set the example for the difficulty that's associated with trying to bring down an Emperor of the sea.
Something else that's worth considering is how to judge fights between two Emperors. There are a handful of these kinds of fights throughout One Piece; Whitebeard vs. Shanks, Shanks vs. Kaidō, and Kaidō vs. Big Mom. However, none of these fights are shown in their entirety, nor are the end results ever shown.
While Emperor battles are never on full display, it hardly matters. Regardless of who wins and who loses, these battles reinforce the idea that Emperors are on their own level of power and cannot be beaten by ordinary people. Thus, their invincibility remains uncontested.
With that said, there are ways to "beat" an Emperor without making them fall in battle. In Shanks' confrontation with Kaidō, for example, the former's win condition was to prevent the latter from intercepting Whitebeard on his way to Marineford. The details of this confrontation aren't specified, but the fact that it's Shanks who arrives at Marineford rather than Kaidō implies that the latter had lost. At the same time, not knowing the details keeps Kaidō's reputation as the strongest creature intact, at least for the audience.
In a similar manner, Luffy could be said to have beaten Big Mom on Whole Cake Island. His intention was never to battle the Emperor but instead to retrieve his friend Sanji and get out of her domain alive. Not only was he successful in doing this, but he managed to humiliate the Emperor every step of the way; he laid waste to some of her strongest children; his allies put together an entire plan that had Big Mom acting exactly as they planned; he crashed a wedding by eating her highly-anticipated wedding cake and destroying a picture of Mother Caramel, her most prized possessions. Luffy and co. did everything short of beating Big Mom in a fight, which is precisely why the papers reported the incident as her loss.
A more debatable loss for Big Mom comes from her attempted entry into Wano. As her ship was clearing the waterfall that marked the entry point, it was intercepted by King and knocked back down to the bottom. Meanwhile, Big Mom lost her footing and fell into the water; she later washed ashore unconscious. If this confrontation had taken place anywhere else, it's likely that Big Mom would have beaten King without issue.
It's also important to understand what exactly happened between Big Mom and Queen. This match ended with Big Mom falling asleep and being captured. While Queen was the last of these two to land a blow, it probably wasn't the deciding factor in this confrontation. In fact, before Big Mom randomly fell asleep, Queen was fearful for his life. Once again, if this were a standard fight, Queen would probably lose every time.
While Emperors are known for being caught in unfavorable situations that lead to their defeat, there are times when it seems like they should legitimately come out the losers. A good example of this would be the battle between Kaidō and Oden years prior to the start of the series. Unfortunately, the dethroned shogun was distracted by one of Kaidō's allies and subsequently taken down by the King of the Beasts. For how things were playing out up to that point, many fans were convinced that Oden would have won against Kaidō in a proper match.
With all of these different definitions for losses in mind, the idea that the Worst Generation "beat" the Emperors becomes a lot more believable. At the very least, the end results of each match could be said to lean in favor of the Worst Generation. However, for the sake of argument, here's a closer look at the outcomes of each match.
The first match to conclude was Big Mom's battle with Kid and Law. The two of them worked together and used everything they had just to drive Big Mom off of Onigashima. Since this effectively removed her from the battle, Kid and Law were declared the victors of this match.
However, there are a lot of ways in which Big Mom could be said to have not lost against the Worst Generation. For one thing, Kid and Law weren't able to kill her or even knock her out, which means that she still had the advantage physically over both of them. Furthermore, she vowed revenge against the two of them; not only does this suggest that she'll be back for another round, but she also doesn't feel all that threatened by them. She likely knows that in most other scenarios, she would have beaten them. Some of these points can also be applied to Kaidō.
Luffy's win over Kaidō is also up for debate. For one thing, it took the Straw Hat Captain no less than three separate tries to get his victory over the Emperor. There's also the fact that Kaidō deliberately took his strongest attack head-on regardless of the consequences. Furthermore, it's debatable whether Kaidō was taken out by the attack itself or the volcanic eruption the attack knocked him into; this same debate can be applied to Big Mom. This is all to say nothing for Luffy miraculously acquiring Gear 5 and his Awakening during all of this. While Luffy technically outmuscled Kaidō on this one, it's possible he would have won under different circumstances, especially if he had fought smarter.
However, the goal here isn't necessarily to beat the Emperors in a straight fight. As has been proven in the past, there are all sorts of means of meeting win conditions against the Emperors that maintain the sense of how powerful they are. As satisfying as it would be to see the Worst Generation truly surpass these beacons of strength, what really matters at this moment is accomplishing their personal goals for these fights.
Whether or not the battle for Wano can be counted as a loss for the Emperors, it should definitely be counted as a win for the Worst Generation, or at least Luffy. He'll be happy simply forcing Kaidō and his forces to withdraw from Wano, which they probably will. While Kid and Law might have felt more comfortable taking the Emperors' heads, they did meet an important win condition against Big Mom by removing her from the fight. In any case, rendering the opponents unconscious rather than killing them has always been Luffy's win condition, so he should take no issue with this outcome. Even if the eruption is responsible for the knockout, they can still take pride in a win by ring out for what it's worth.
The Worst Generation's victory might not be the most ideal, but it's a victory nonetheless. They staked their pride against the Emperors' pride in a fight and ultimately came out on top. In this sense, they can honestly lay claim to having "beaten" the Emperors of the sea.