Don't want to dunk on that person specifically, but the idea that people that look black to us in Medieval art prove existence of black people in Medieval Europe is bad on a lot of accounts, but one of it is that Medieval art isn't known for its realism. Most of the time it was drawn from description alone, and even when an artist specifically wanted to make their skin different they rarely knew the actual skin tones. Like I think I saw a man on Medieval illustration that was literally black as coal and he was supposed to be an Arab. Luke the artist just read "Arabs have darker look" or something like that and drew what they imagined it to be like. And there are other things about Medieval art not being very realistic.

Which is not to say that there weren't non-white people in Europe. For one there was like a lot of Arabs and other people from SWANA all across the Mediterranean coast with some individuals travelling anywhere, and people from any part of Eurasia or Africa could (and did) just come there, but like search actual proves.