I hear this a lot, particularly in arguments from the religious right. I understand that On the Jewish Question is very confusing to read because of how the word "jew" was equated with "usurer" in the 19th century leads to this conclusion - this is often used to discredit all of Marx's work.
Was he, or is this another case of misinterpretation?
You know how it was socially acceptable like 10 years ago for people to say stuff like "this class is fucking gay" and 10 years before that it was pretty normal for guys to call each other f*gs for liking a girly song? Well 200 years ago, even if you weren't someone who wanted to expel/kill all Jews/minorities, latent antisemitism and racism were ubiquitous. Even if Marx was Jewish himself, and was an admirer of the abolitionists in the US, he would still do shit like call LaSalle a "Jewish n----r". People really just didn't care about being offensive, and stereotypes were extremely common and accepted as truth.