“A nation’s greatness is measured by how it treats its weakest members.” Immigrants are non-citizens are among the weakest members of American society (an outsize number of laws that impact them and zero capacity to participate in its political machinery)
Non-citizens aren't members of the nation, so the US are okay if we base morality on that random quote.
Let's say North Korea selected a few of the leading family and declared they are "members", under such circumstances would that quote imply that North Korea is the greatest nation, because all its "members" have near King-like status?
Or likewise, can a nation increase its greatness by declaring the weakest among them to be "non-members".