I think that's a bit harsh. In the case of the USA my impression is that the main source of cultural diversity is from immigrant communities whereas in Europe the main diversity is between long established communities - often at a level well below that of the current nation state (particularly noticeable in places like the UK and the other larger European states).
Personally, I think that Europe does have a lot more cultural diversity than the United States - but I can see that the point is arguable and that it's hardly a case of being "delusional" to argue the opposing point.
True, but then I'd go so far as to add that the USA is more diverse than any one country making up the EU.
And if that's the case, treating the USA as a comparable EU country in terms of diversity ignores that diversity found here. Which is what was being discussed in the first place.
Personally, I think that Europe does have a lot more cultural diversity than the United States - but I can see that the point is arguable and that it's hardly a case of being "delusional" to argue the opposing point.