> Is the not X it's Y in large frequency not an AI tell?

I doubt it. The AIs are statistical models, if they've picked up a habit of saying "not X it's Y" then that is probably the most likely thing for humans to say when they are explaining something. The whole training process is about making what the AI says statistically indistinguishable from what humans do; the only way to pick up that it is an AI is either because the model is badly fitted (which, in fairness, many are, they're still working out the ideal weights) or because it isn't grounded in reality. It isn't reasonable to say "oh this looks like AI" based on small phrases like that, AIs use the same phrases humans tend to. It is where they should be doing the best job of fitting in with us.

"This uses really common phrasing ergo it is AI" is a bad case to be trying to make.