I think the question about personal experience with black people was due to the fact that there are things you just don't realise if you don't have first or second-hand experience of them.
For instance, you might not think that people in a wheelchair are inferior, but you might also not realise how impractical some things are for them. You could, say, build a restaurant and fail to make it easily accessible, which while not ableist (no hatred against the non-able bodied, no negative intent), is still de facto discriminatory, in that they just can't eat there even if they want to.
If you're not part of any minority and have also never spent time around them, it's possible to be completely unaware of many little everyday things you take for granted that are different for them.
Now, with all this being said, I'm not quite sure how that's relevant to how many non-caucasian people we know who practice poly, but I don't think it was suggested that everyone who isn't surrounded by people from every possible minority is a racist. Simply that they might not be aware of some things and should keep it in mind.