I think in this instance it means "known to the Mongols"

"Known world" in the context of history usually means Eurasia + Africa. Basically all the places that aren't the new world and random islands in the middle of nowhere.

Known world does not usually mean Africa. It generally means Europe and central Asia... because that's the known world to European explorers.

Oceania, Africa, the Americas are not included in that nomenclature usually.

Considering Rome ruled a bunch of Africa I'm pretty sure they knew about it...