What he meant by “ludicrous position” he didn’t specify. But in context, “them” can only denote “Jewish people”.
He went on: “For example, the other evening we had a meeting” where the Palestinian Ambassador spoke. It was “dutifully recorded by the thankfully silent Zionists who were in the audience on that occasion” who then berated the Palestinian Ambassador for what he had said.
“They clearly have two problems,” Mr Corbyn said. “One is they don’t want to study history. And secondly, having lived in this country for a very long time, probably all their lives, they don’t understand English irony either.”
In short, the incident is told as an “example” of the “ludicrous position” represented by Zionism as a whole.
Perhaps the words did not come out quite as he intended - but that is hardly the fault of his listeners.