Agreed about the others but wouldn't the Okies be part of the Scots-Irish strain he covers?
There's a reason why bock bier (eg. Shiner Bock), dryland farming (a Russian German thing), frybread (from the Bureau of Indian Affairs era), Cowboy culture (a merger of older Hispanic Ranchero culture with southern settler culture), etc became a thing in Texas and Oklahoma
(and to your point about italians: I'm not sure but suspect the use of snubbing posts in round pens came via swamp italians. Also, don't forget the portuguese: not as prominent as on HI, but still pretty common, eg Devin Nunes)
During my 20+ years in the Bay Area I listened to a lot of Portuguese/Azorean radio, so much that it corrupted my accent. I was born in the Texas of Brazil but I sound a bit like the British equivalent now.
Absolutely (though it's specifically Azoreans)!
And to that point, stereotypes do not take into account the historic diversity that Central California has had.
Mexican, Okies, Portuguese/Azorean, Italian, German, Czech, Armenian, Chinese, Punjabi, Russian, and other settlers had an outsized influence on the development of Central California.
And this itself highlights how Albion's Seed does not generalize well outside the Eastern Seaboard (nor did Fischer intend for his work to be applied that way - he only concentrated on New England, Virginia, the lower Delaware Valley/Philly, and Appalachia, not on the rest of the US.