Yeah, style attribution is something big generalist models are usually pretty bad at, even on the material they have likely been trained on. Sure they are classifiers but this ability is limited, there's too much going on in them and they aren't magic. This needs a proper experiment, not anecdotal evidence.

Or it needs a minimum sample size in the training set. I fed it some small private conversations and it refused, then some heavier ones and it guessed John Carmack. Flattering but wrong.