I probably shouldn't have used the word "standalone"...I simply meant that it's an original story in their N52 inspired animated universe instead of the faithful adaptation they told us we were getting. So in that sense, it's a "standalone original film in that universe" instead of "faithful adaptation".
Don't get me wrong, I also loved this film. I think it's great! But I am hung up on what they announced it as...if they had called this "The Return of Superman", I would have gone in from the get-go with zero expectations. But they called it "The Reign of the Supermen", which comes with baggage and expectations. When they announced this at Comic Con, they made it sound like we were getting the Superman equivalent to The Dark Knight Returns; instead, we got the Superman equivalent to Throne of Atlantis or Batman vs Robin...those films had elements of the comics (Throne of Atlantis and The Court of Owls, respectively), but were done in a mostly original way. Death of/Reign was billed as a faithful adaptation & instead we got original stories with some elements of Death/Reign. Nothing wrong with that on the surface, and I dug the Hell out of them. They just weren't what I imagined when they were announced.