Arthur Koestler would have agreed, his “The Thirteenth Tribe” elaborated on the Ashkenazi - Khazar theory. I think this idea gets hotly disputed but I have no opinion either way.
I read two of his other books, on his break with Communism, and found Koestler to be a good writer. ‘Arrow in the Blue’ and ‘The Invisible Writing’ IIRC.
https://www.amazon.com/Thirteenth-Tribe-Arthur-Koestler/dp/0945001428
Israel genealogists who really take this stuff serious don’t dispute it unless neo Nazis hijack the data
No question the King of Khazaria converted to Judaism and much of his territory did too
Where Sephardic oriental and Ashkenazim all evolved from is speculative though I have no doubt given their fixation on maternal identity that they sure have some ties to Judah
What percentage who knows