Presumably the "subject matter expert" will review the output of the LLM, just like a reviewer. I think it's disingenuous to assume that just because someone used AI they didn't look at or reviewed the output.

A serious one yes.

But why would a serious person claim that they wrote this without AI when it's obvious they used it?!

Using any tool is fine, but someone bragging about not having used a tool they actually used should make you suspicious about the amount of care that went to their work.