I don’t know how white Bohemia was in the 1400s, I do know that the multiple arguments against it being white revolved around there being black people in other parts of Europe that were thousands of kilometres away and had had strong links to north africa/middle east for centuries anyway. Looking at the medieval POC blog, they’re almost entirely Italian art works, or art from other European countries depicting events outside Europe.
I can see that the designer is a bit of an asshole, but I think people underestimate just how different Eastern European societies today are, particularly in the areas of race, sex and gender. Also, PoC are still incredibly rare in Poland, Czech Republic and Lithuania. So rare as to have people stopping and staring.