The simplest answer is that he had very little connection to his Jewish heritage as a cultural Christian who was very assimilated with white, Christian Germany, and had some elements of personal chauvinism. While he opposed antisemitism as a political movement targeting Jewish people and blaming them for societal ills, that doesn’t mean that he liked Jews personally.

Another possibility is that he said it without any particular concern for racial ideology because it’s just something people said, and he was callous to the manner in which using such terms reinforce racial ideology. Like how some people say b***h but don’t particularly have the thought in their head “this dude is like a negative example of a woman” but are just following the surface level social meaning of “coward,” which is still a bad thing to do because it’s inconsiderate of how the language is harmful, but reflects a different internal attitude.

I’m not a philologist at all, let alone for Marx’s time and place, so I can’t really evaluate whether it is one of these or something else.

I do think it’s a significant element that this is like the only known case of him saying this (and it was somewhat casually in a personal letter), and that’s not for a lack of personal correspondences from him. We have a lot and I think this is the only case. I think he was just being flippant.