It's exactly how it works - at least potentially. Lean text is harder to watermark because word choices and meanings are tightly constrained.

Low-entropy text is fluff and filler. It's very easy to synonym-substitute words without changing the message - if there even is one.

Perhaps, but there are certainly now catchphrases and words that can indicate it was written with AI i.e. load-bearing, idempotent, etc. Style and structure are in and of themselves, a fingerprint.