If you look to the west from Europe you see a vast sea. It doesn't take a great deal of effort to imagine some form of land beyond or in that sea. You would be quite a boring and unimaginative person to not wonder about this.

Edgar Allen Poe's novel The Narrative of Arthur Gordon Pym of Nantucket details a fictional account of what might be in the Antarctic, an unexplored area in his time. We now know that Poe's imagined account is a far cry from the reality of the Antarctic continent, but he could have been "right" in that there are people living there. That doesn't really imply any knowledge of such lands though. If Poe had lived several hundreds years earlier he might have written a similar story with s/Antarctic/across the Atlantic/.

Add a few hundreds years with confusion between "fiction" (or "myth" or "legend", if you will) and "science" (a concept which didn't really exist in the first place, at least not in the same form) and things get very murky fast.

I don't think that the mere existence of the concept of "Antillia" really proves any actual knowledge; there needs to be some additional evidence; reading that Wikipedia page there doesn't seem to be any. We'll likely never know for certain if the roots of Antillia were based in reality or entirely fictional.