He was anti-zionist sure, but he also openly associated with and supported groups that go beyond anti-zionism into full blown anti-semitism and, despite plenty of prompting, continuously maintained that there was no issue with this.
There’s inferences made there, sure, but it’s very easy to say “I generally support X but they take Y too far” - even the current government is doing it when supporting Israel, but also making (hollow) statements that they should be more careful not to attack civilians as it’s clearly not right, but they still want to support their allies. Corbyn could’ve made similar statements, but he didn’t, either meaning that he didn’t see anything wrong with the anti-semitism or he thought that them also being anti-zionist excused it, neither of which are a good look.