> Certainly someone who immigrated to the US as an adult cannot know more about the US than someone who grew up here.
They certainly can, might even get a Ph.d in Cultural anthropology albeit without necessarily grasping small local intricacies, but the same could be said about someone from a different state.
Though US here is a bad example, it's just too culturally heterogeneous compared to China.
Certainly someone who immigrated to the US as an adult cannot know more about the US than someone who grew up here.
> What sort of claim is this?
A fairly straightforward one.
I don't think the guy's comment was that controversial or baffling.