Wouldn’t the French classics have been part of pangram’s training set? That doesn’t seem like strong evidence at all.
Comparing to other Haitian authors was also relevant, because another defence from the author was that his "caribbean style" could have tripped up the detector.
But yes, other than that, that's a good point. My anecdotal evidence is that my own unpublished writing, both in French and English, contains no false positives, but even in that case, that's only evidence that my style isn't AI-adjacent. I'd be curious to see concrete examples of false positives, but it's hard to fully trust these if they don't have proven timestamps.