You absolutely do not need AI in order to sound different in one context versus another. I mean, I highly doubt most people on Lemmy speak to their bosses in the exact way that they write their comments here.

Hell, I'd be surprised if they spoke to their friends and family the same way all the time (yes, I'm aware that you can generally be more lax around friends - but there's a time and place for it, whereas comments on message boards tend to just be lax all the time).

That very concept has been around far longer than "AI" has.