If it's not an LLM, I guess the author spent too much time around them, because it totally reads like an LLM. And that's not a good thing.

Which bits read like an LLM?

I didn't do any "it's not X, it's Y" or other classic LLM tropes as far as I can tell.

Might be the numbered list of short points that keyed OP. And it's actually a great communication style! So much so that they overtrained LLMs to produce them. I wonder if we're all going to have to get idiosyncratically slightly worse to avoid the suspicion in the future.