That doesn't give you the right to call it "ultimate" [stressing the pure meaning of the word]!
These theories of mine are Manga-based, dude. The Demon King, Merlin, and Elizabeth all have seen Meliodas going AM, and they all confirmed AFTER the fight that he's STILL reverting to his Prime self
He was getting stronger but his form didn't change. It is still Assault Mode, there is nothing beyond this besides Indura, nothing.
He is currently changing because of the absorption, but its something which never happened before, maybe his form won't change at all.