I think most are thinking about a higher proportion of adults living at the same household as their parents in some countries when they say that. However in recent year the proportion in the US got a lot closer to Latin American countries. Then again it probably significantly varies by race, ethnic background etc. which doesn't invalidate the anecdotal evidence people might have.

> cultures with such a broad brush weird, especially

US is very heterogeneous but it works reasonably well in many other places besides a handful of outliers.