Is a Frenchman who migrates to the U.S. at the age of 12, who is fully educated there and spends the rest of his life and 70 years of writing-teaching life there still a 'Frenchman'?

Yes. He's also a good writer and a superb critic. He just doesn't know squat about linguistics, so he should keep his misinformation to himself.

Ceci n'est pas un seing.