Memes

from a Sino-Roman perspective africa was the second continent/world “discovered”

Is that even the case, though? Certainly not for the Romans (to whom Africa may as well have been next door, whose early rivals were the Carthaginians, and whose writing system ultimately originated from the much more ancient Egyptian hieroglyphs).

And the Chinese trade contacts with Africa, while originally indirect, were such that Africa was not exactly unknown to them (and Zheng He's voyages were more about establishing direct contact/etc). Distant and mysterious, maybe (like how Rome and China saw each other for most of history) but not "undiscovered" by any means, and with continuous and ancient trade routes.

Could you provide a source for that claim?

SadArtemis already expanded on this well. Certainly Greco-Roman societies were well aware of Africa, even if not the extent of it. You might find Wikipedia's early world maps page a good starting point.