While this is a technically correct answer, I can't help but think the US isn't at particular risk of being colonised by a militarily overwhelming foreign force that also has smallpox. Indeed, if this were a concern, mere immigration law would hardly help!
Technically speaking, the US is the overwhelming military force that also has smallpox (in a BSL-4 lab somewhere), so perhaps his warning was for other countries, just in case?
But the point about the immigration law probably not helping much in this scenario still stands.
Are you seriously unaware that that's "not a thing"? No US government entity, military or otherwise, has ever deliberately infected a native population with smallpox. You're reduced to making shit up?
I don’t think any Native American societies lost their lands, language and culture, because they had free migration as a policy. Rather they lost it because of an active genocide conducted by a hostile force.