You’re not insane. This is definitely AI.

In what sense? The phrasing is just a generalization, production-grade anything needs consideration of the needs and goals of the project.

“<x> isn’t just <y>, it’s <z>” is an AI smell.

It is, but partly because it is a common form in the training data. LLM output seems to use the form more than people, presumably either due to some bias in the training data (or the way it is tokenised) or due to other common token sequences leading into it (remember: it isn't an official acronym but Glorified Predictive Text is an accurate description). While it is a smell, it certainly isn't a reliable marker, there needs to be more evidence than that.

Wouldn't that just be because the construction is common in the training materials, which means it's a common construction in human writing?

It must be, but any given article is likely to not be the average of the training material, and thus has a different expectedness of such a construction.