The merged Superman appeared earlier in the issue on DC's main Earth at a funeral, so it's almost certainly not him. As its name suggests, the Dark Multiverse is full of dark, twisted versions of DC heroes. While this Superman seems like himself so far, he could just be a dark Superman who hasn't turned evil yet. He could also be a creation of the story's villain, who was absorbing energy from the Unseen for a mysterious purpose.
However, it seems like this Superman might be the real deal, especially since Grant Morrison is credited with co-writing the next part of this story, Sideways Annual #1. This Superman could be some kind of remnant or echo of the character who was merged in "Superman Reborn," or he could hail from a world based on the pre-Rebirth New 52.
Either way, it's worth remembering that the Dark Multiverse isn't just a place where a bunch of evil Batmen live. It's also the place where worlds built around unstable ideas die in an endless churn of destruction.
While the New 52 was a sales smash with plenty of critically-acclaimed highlights, some aspects of that reboot simply didn't work. The New 52's younger Superman was hardly the worst part of the relaunch, but he was still phased out in favor of a more traditional Man of Steel. When the New 52 Superman's history was erased, all of that time had to go somewhere, and the Dark Multiverse seems like the logical landing place for those adventures.
In a lot of ways, this Superman is the embodiment of the New 52. Since he and other parts of the New 52 weren't able to succeed in the real world, this entire storyline could be read as a meta-commentary that acknowledges the parts of the New 52 that didn't work by throwing them in DC's multiversal scrapheap.
During Morrison's seminal run writing Animal Man, Morrison waded into similar territory with Limbo, a part of the DC Universe that held long-forgotten characters who were put on a metaphorical shelf. Since he was one of the primary creative forces behind this Superman, it makes sense for Morrison to send him off with one final blaze of glory before obliteration.
No matter who he is, it doesn't look like the Superman will be around for long. Going by the covers DC has released, it doesn't look like this Superman will appear anywhere after the Sideways annual. Even though this Superman says that he's "never being put away again" in Sideways, this could very well be the New 52 Superman's final adventure.
Sideways Annual #1 is written by Dan DiDio and Grant Morrison, with art by Max Raynor and is on sale on October 31.