You don't need much long-form writing for your idiolect to be identifiable. I recently returned to Reddit after a break and see that people generally don't write very long comments now, except for the autistic and others who can't follow social cues. But I already noticed that my short comments occasionally use turns of phrase that are fairly peculiar to me. Should be possible for a social media giant or nation-scale actor to deanonymize me. And some people give themselves away by outright posting in country or city subreddits.