In my personal experience (some of it on the receiving end), the "accusations" of "preteciousness" usually say more about the accuser (and their set of life experiences), than the accused.
The only person who could objectively make that claim is someone who is very-well-versed and up-to-date with the in the targeted writer's writing and/or speaking style.
That is to say - I perceive preteciousness to be something an author or orator purposefully and/or intentionally does for the specific occasion and/or audience, this being "unlike themself" for potentially spurious or considered-bad-by-the-accuser reasons. If that is the "natural" way the person in question communicates, I'd consider the claim false.
[As mentioned, from personal experience, the "new" people ("barely-an-acquitances") often mark or accuse me of it, on account of my (unintentional) code-switching and/or quite consistent mixing of my native language and English, especially when it comes to writing. Meanwhile, people who have known me for years know it to be just one of my (many) idiosyncrasies.]