The perplexity point deserves a warning label of its own. The 2023 Stanford study (Liang et al., "GPT detectors are biased against non-native English writers") ran seven detectors over TOEFL essays and got an average false-positive rate of 61.22%, while essays by US eighth graders were classified almost perfectly. Their explanation is the signal you list first: the flagged essays had significantly lower perplexity, because plainer vocabulary reads as "smooth".

A 2026 re-test (Al Ali, Helcl and Libovický) found the English rate down to 23.1%, and no systematic bias against non-native writers of Czech, so the effect seems to depend on the language. Still roughly one essay in four in English.

So per-length calibration is a good start. I'd add a line to the verdict whenever low perplexity is doing most of the work, saying plainly that second-language English triggers it too. Turnitin's own guidance is to use its score "to initiate a conversation, not to draw a conclusion", and showing which signals fired is the part that makes that conversation possible.