It depnds on how we interpret what Grant Morrison and other wrrters have implied.
How do you interpret the interpretation by Geoff Johns that everything come from Superman?
Superman is the idea that won die. When asked what should be written on his tombstone, the words “To Be Continued”. That’s basically Superman. It’s been over 80 years since he was created and he is still an extremely popular character. DC embraced that idea and kept saying it again with Doomsday clock and a few other big issues the last few years
CAS is powered by this fundamental idea. That the Superman story is the strongest story. That even when the comic itself is being eaten alive, or even forgotten, the idea of Superman persist.
Again you may not agree but DC has given enough hints through the years. There is a reason even the strongest reality warping can’t change Superman. Hell even Mxy or dr. Man can’t change Superman forever. They can alter stuff about him, when his parents die, if he has kids or wife..etc, but the fundamental idea always comes back
That is why even someone like Lucifer can’t beat CAS.