It seems pretty ridiculous that people are delving into tweets from 12 years ago that were perhaps a bit lacking in taste or humour but were not (as far as I can see) anti-Semitic. Still, if someone is not Jewish (and Delaney's not) but spends a lot of time making jokes about Jewish people, it does make you wonder why that person makes a lot of jokes about that particular religious/cultural group.
It's also worth noting that something that may seem inoffensive to people who do not experience abuse or discrimination, can seem different to those who do experience abuse or discrimination.
So we can look at a comment about Jewish boys being circumcised and getting their horns filed off at the same time and think, "Oh, that's pretty silly, but he's just riffing on the idea of Jews being evil and ridiculing the idea". A Jewish person may understand that Delaney is ridiculing the idea - in other words, they understand his intent - but still be unhappy as they're concerned that a) not everyone will get the ridicule and b) it's reinforcing, or emphasising, the idea that Jewish people are evil. So even though it's not agreeing with a stereotype, it is still publicising the stereotype, drawing attention to the stereotype.