He's not talking about other nations, he's talking about the US and saying if you are not a citizen of a nation, its a foreign nation to you and they have no obligation to let you in.

> Foreign countries have no obligation to admit you within their borders.

This is obviously a general statement about any nation, comparing the US to its peers.

In the context of the conversation it is clearly an argument that “we don’t have to let you in, we can require whatever we want, including trampling on your rights as an individual”, which is unamerican.