So I guess ultimately my question is why are we assuming that Nayuta specifically meant “Denji belongs to me definitively, and you are trying to take his attention away from me?”

Aside from the fact that narrative misdirection is a thing and that I think it would completely contradict all the themes of the story thus far to just have her be Makima 2.0 and inherently evil...

We don’t actually know if she and Yoru recognize each other or not? Even on a subconscious level. If she does recognize Yoru (and, reasonably, knows how her powers work) calling Yoru a thief could have meant that “You are trying to steal Denji’s spinal cord to make a weapon” or even “You have stolen this random girl’s body to use for yourself.” (Even if she doesn’t know specifically that Yoru is sharing Asa’s body, she might still be able to tell that something weird is going on?)

She’s still a child, she might have just impulsively said the smallest amount of words that would sort-of convey what she was feeling.

I’m not worried yet.

Yet.