You’re conflating two theories: one widespread, and one fringe. The theory that the Ashkenazi population experienced a bottleneck is widely accepted; claiming that Ashkenazim are actually a remnant of the Khazar Khanganate[1] is both fringe and typically associated with antisemitic conspiracies.
(Note that I say antisemitic, and not in a manner that involves conflation with Zionism: going against the overwhelming majority of generic evidence to make a claim about a Jewish ethnic group that doesn’t even majority reside in Israel reeks of a blood-and-boden anger against Jews because of who they are.)
(Note that I say antisemitic, and not in a manner that involves conflation with Zionism: going against the overwhelming majority of generic evidence to make a claim about a Jewish ethnic group that doesn’t even majority reside in Israel reeks of a blood-and-boden anger against Jews because of who they are.)
[1]: https://en.wikipedia.org/wiki/Khazar_hypothesis_of_Ashkenazi...