The scale is very much subject to discussion, but genetic evidences[0] support the core idea.
If it really is a joke, what is the punchline? How does it work? What's being made fun of? That it would be outrageous for someone to say that seriously? As "red pillers" like to say: keep making jokes until the joke really lands (i.e. accustom people to hearing it by framing it as a crass joke so they'll no longer flinch when it is said in earnest).
How is it antisemitic ? Are Khazars secretly running the world from Atlantis ?
Besides, you know that Khazars are not alone? They are supported by the Nazis from the dark side of the moon. "They" just don't want you to see this truth. But they are all just servants of the lizards from the Earth, who only leave their habitat when they show up masked as a royal family in Buckingham Palace.
Should I put a :) or a /s or some other sign to make it clear it's a joke or sarcasm?