It's an interesting point of view, even if it's not the mainstream political-science meaning of "nation state". Certainly the USA has a strong national identity, shared traditions, a couple of nearly unique dialects (though GA and AAVE are also spoken in Canada), and a state religion in which schoolchildren are forced to pray daily to the Flag, and to a significant extent the USA grew out of a tribal invasion and colonization of America by English colonists with shared descent.

Still, I think the US is better understood as a multiethnic, profoundly racist society, not a single tribe: 41 million people (12% of the population) speak Spanish, another 13% speak AAVE, and 2% belong to various Native American and Alaskan Native nations. In all cases most speakers also speak the language of the dominant GA-speaking tribe, and there are identifiable musical, culinary, religious, political differences associated with their varying descent, along with striking segregation in housing, schooling, and education. Subsequent to the initial English colonization there was also substantial immigration from Ireland (9% of the current population), Scotland (8%), Germany (15%, already 9% by the first census in 01790), the Netherlands (1%), Italy (5%), and Poland (3%), China (1.5%), India (1%), France (3%), as well as other countries. Some of these immigrants were also Romani (0.3%) or Jewish (3%). But, in the US system of racism, these differences are largely submerged in generalized "[non-Hispanic] white American" and "Asian American" (7%) ethnic identities.

Since only 62% of the US population belongs to its hegemonic "non-Hispanic white" nation, which is as you say built out of immigrants from various European countries, I don't think it's reasonable to describe the USA as a nation state. It's not even officially white supremacist, although its historical foundations are in genocide and slavery, and in practice its government treats its ethnic minorities very badly indeed even today. It's at least two more major genocides away from becoming a nation state, although Obama's horrifying mass deportations of "illegal immigrants" and their weaker continuation under Trump were a significant step in that direction, as is the ongoing GULAG-scale mass imprisonment of mostly African-Americans, many of whom are enslaved in prison.