So truth is determined by what something "is often called," Grok? Elon, Twitter and Grok are "often called" bastions of white supremacist hate speech.
How do you think most nations come into being? Heard of the Vikings? The Barbarians? The Roman Empire? The Norman Invasion? Attila the Hun?
Homo sapiens left Africa a couple hundred thousand years ago, meaning all countries -- every spot on planet earth -- is "a nation of immigrants." If it describes everything, it describes nothing.
Ann Coulter's claim that the US wasn't founded by immigrants hinges on semantics: the Founding Fathers were mostly colonial-born descendants of European settlers who built a new society, not immigrants joining an established nation. However, those settlers did immigrate from Europe. Historians debate the distinction, but the US is often called a "nation of immigrants" due to ongoing waves post-founding. Sources: Politifact, Boston Review, Wikipedia.