Moris ties to the world get cut upon entering the paradise (the Nirvana realm) which he only does 1 whole fight after "becoming the world". So Mori in the final fight with Mujin became the world, then ascended beyond that form to an "absolute existence" afterwards. Meaning the 2 don't actually contradict themselves.
Because when he ascends to that place is when his body changes. The context behind the whole place is this:
People who achieve Nirvana end up in the paradise (or heaven, the Nirvana realm).
When they decide to stay in the paradise beyond Nirvana they become a Buddha - an absolute existence and an all powerful spirit who perceives all of creation in a form in Golden tablets.
So it's not that him being in that place results in the power to oversee all of creation, but the other way around. His power is what results in him being in that place.
That actually factually
CAN'T be the case because it's not just Mori who became the world. THEY as in Mori AND Mujin became the world and Mujin not only never achieved Nirvana or entered the Nirvana realm, Mujin straight up died at the end of the fight with Mori only after which Mori entered the Nirvana realm.
That is definitely not the case. It's due to nirvana itself.
All that's in the Nirvana realm are these golden tablets.
We see that when Mori interacts with them what's displayed on them is directly affected (moving a golden tablet completely shakes up the space inside and inserts a silhouette)
Not only is this point in space but also a point in the past as this is an event we've seen happen on screen exactly 60 chapters before this scene.
In other words, "all of creation" here are clearly the golden tablets and is referring not just to the universe but the entire timeline itself. This is something that can only logically be a result of his physiology changing as he can easily grab, move, and control these tablets when while he wasn't in the Nirvana realm he was actually within them. This makes sense as the Nirvana realm is described to be the "place where time and space overlap" or "where time and space are nested" (depending on translation).
What I'm getting at is this: Imagine you have a book. You can interact with it, rewrite it, move it around, etc, thanks to your physiology. But if that book was in another room, you would need to enter that room in order to do so. That doesn't mean the room is what gives you the power to interact with the book. It just means the book is in that room. I'll make a TLDR that explains this in a more simple way shortly but I felt like giving the full context.