this is a well-studied field (stylometry). when combining writing styles, vocabulary, posting times, etc. you absolutely can narrow it down to specific people.

even when people deliberately try to feign some aspects (e.g. switching writing styles for different pseudonyms), they will almost always slip up and revert to their most comfortable style over time. which is great, because if they aren't also regularly changing pseudonyms (which are also subject to limited stylometry, so pseudonym creation should be somewhat randomized in name, location, etc.), you only need to catch them slipping once to get the whole history of that pseudonym (and potentially others, once that one is confirmed).

People do change over time, I used to write "ha" after every sentence for some reason