This.
The 21st century understanding of "whiteness" is not at all what it was understood to be centuries ago. Today, an Anglo, an Italian, an Irishman, and sometimes even a Jew can all be considered "white" in the United States just because they aren't "brown." In the 19th century, There is no way any of the latter three would be considered "white." Catholic immigrants were viewed as a threat to the United States on a political, spiritual, and racial level, which is why the Know-Nothings and eventually the second iteration of the Klan opposed immigration from those countries. Why? Because "wops" and "potato niggers" brought crime and tended to ghettoize. Most of the stereotypes applied to Muslim migrants were applied to Catholic immigrants (and not without cause). No way would Nicky Fuentes be considered a good, white American in the 19th century.
This is why this pan-white activist LARPing is so cringe. It has little to no basis in history. The West's best hope is to re-establish older national identities (e.g., Brexit), and European populists seem to have the best chance of salvaging the nation state. The Spencerian ethnostate could only be conceived by someone incredibly naive of European and American history.