> I don't know of many mainstream Jewish historians who would agree that antisemitism didn't exist prior to the 19th century.
Depends on your your definition of "antisemitism"; if you mean "general prejudice against Jews", then sure, obvious that existed. But if you you mean "anti-Semitism against the Semitic race (as opposed to the Aryan race)", then that's quite a different thing.
I don't think it's a hard-line distinction; obviously there's overlap and nuance. But the move from more or less generic religious persecution to racial-based persecution was a very marked and notable shift that many many people have commented on, and that's really not very controversial.
Does this distinction matter? Well, it seems to me that it does. I don't think the holocaust would have happened without this. And all of this strongly shaped Nazi world-views, which was really the point I wanted to make.