But most writers write about things they've seen--even fantasists often have characters based on people they've met. Assuming that sociopaths have existed all throughout history, then even if for much of human history no one knew what a sociopath was, it's likely there were authors who wrote a depiction of a sociopath just by writing about their old rival Timothy or their father-in-law Paul. In that sense, we at least can't rule out that there are fictional characters from a time before sociopathy was a known condition who nonetheless are "sociopathic" characters.

(Actually, I think this is what the previous poster meant, the one you said was "speaking in code." I hope this was clearer.)

Edit: I saw your post below about them only being able to identify "sociopathic traits." Does the dividing line really matter? A post above mentioned monomania--that's no longer an existing diagnosis, but an author who wrote about monomania can still be recognized today as writing symptoms of currently defined illnesses.