I think when it comes to how to handle past continuity there's been two schools of thought that have been in near-constant conflict for the past several decades (or even earlier). There's the school of thought which treats every era/version of the character as a distinct continuity/universe/timeline. This was how the original Multiverse concept worked before COIE, and its the philosophy reflected in something like Convergence. And then there's the school of thought that these are all the same characters they've always been, this is the same universe all the way back to stories published in 1938, and they've all just been rebooted or subject to some kind of reality/time manipulation...but in fact ''everything happened''. This was the philosophy behind DC Rebirth and Death Metal. What makes things even more confusing is that often the two schools of thought co-exist at DC at the same time!
The beauty of Doomsday Clock's explanation was that it took the messy co-existence of these two schools of thought at face value, and found a way to make it work. So this is the same universe all the way back to Action Comics # 1. But everytime the universe got rebooted, a 'backup' of it was created preserving that era/continuity, so it was still out there and continued to exist as a separate entity. It allows DC to have their cake and eat it too. The current Superman can be the same guy from 1938 lifting that green car. But you've also explained how the Earth 2 Superman could be a different person, and how the New 52 Superman could theoretically still be out there.