Right, I largely agree with that. But there's a reason I specifically differentiated one's "particular campaign" from the "default" of D&D, which is nothing like 13th century Iceland.It does impact on things though.
If my game rule setting is basically 13th century Iceland (or fantasy fascmile) then having characters of most of the world's ethnicities is a real stretch.
Not that think it's necessarily wrong to make a setting based on 13th century Iceland, but you do need to recognise that it's not inherently the most inclusive setting.
This matters because there's more to a setting than just a world. If I divide my setting into China continent and fantasy scandinvaia type continent and put them on opposite sides of the world then technically I've got some level of diversity, but if I haven't built in any ongoing interaction it's again not going to be particularly inclusive.
In order to have a wide variety of different ethicities represented you do have to have some kind of mixing pot set up. You need to have not just different ethnicities, but some kind of set-up that makes interaction plausbile.