Culture is not always genetically based though, so this is not addressing the term multiculturalism but moreso diversity via race.
Cultural diversity will always be a positive thing until a culture is ready to step forth under scrutiny and declare that theyre perfect.
I would go as far to say that culture is never genetically based. If I were to adopt an African child upon birth I am certain that the culture of his/her genetic parents would travel along with him/her. But I don't think that boedicca was alluding to culture a genetically based. I think boedicca was criticizing those who think genetics are a factor.
So how is cultural diversity a positive again?
Boedicca was answering your OP in regard to cultural diversity as a strength or weakness, by saying that that cultural diversity in and of itself is divisive groupthink.
He didn't bring up DNA just at random, he was referring to your OP quite obviously. And, he is wrong.
And I already stated at the end of the very post you quoted why it's positive. Feel free to read it again and take it all in this time.