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.
Which people do you mean?
> Sandy Island (sometimes labelled in French Île de Sable, and in Spanish Isla Arenosa) is a non-existent island that was charted for over a century as being located near the French territory of New Caledonia between the Chesterfield Islands and Nereus Reef in the eastern Coral Sea.[1] The island was included on many maps and nautical charts from as early as the late 19th century. It was removed from French hydrographic charts in 1974. The island gained wide media and public attention in November 2012 when the R/V Southern Surveyor, an Australian research ship,[2] passed through the area and "undiscovered" it. The island was quickly removed from many maps and data sets, including those of the National Geographic Society and Google Maps.[3]
[0] https://wikipedia.org/wiki/Antillia