Is it a hideous insult because you think it's not true, or because the wording feels offensive? Is there a more polite way to express the same sentiment, if you think it's true, or is it either true or insulting?

Cuba has had zero immigration for a long time but has an interesting culture. Vietnam has basically zero immigration. Indonesia. Philippines. India. Honduras. Guatemala. Brazil. Jamaica. Mexico.

It's both insulting and untrue in a way that feels degrading to these nations' rich thriving cultures. That somehow only western, immigration heavy cultures are valid or are cultures of any worth.

I do think those nations have rich, thriving cultures. I also think that any culture, no matter how rich and thriving, can lead itself toward stagnation if it becomes overly insular. It's fair to point out that immigration isn't the only possible source of cultural diversity, but it's a powerful force for it, and I think the United States, being a huge cultural exporter, is at more risk than countries that are less dominant on the internet.

Just say what you really mean "white people have no culture"

Let me fix that for you:

"The kind of white people that are over-represented in media and mainstream discourse have no culture worth emulating."

Considering that they're, ya know, overrepresented, I can see where it's easy to make the mistake of generalizing to all of them.

This is just racism. Which, to be clear, white people do not experience with any kind of comparability to other races in the West.

Of course that doesn't make it less harmful to commit.

Would it make you feel better if I told you I think Japan, South Korea, and China are at risk of the same?

I'm not worried about which culture, I think any culture that's not exposed to other cultures is putting itself at a disadvantage.