Sometimes his positions have cross over with mainstream progressive positions like anti-apartheid or opposition to the Iraq war. But often he's been in a tiny minority (Falklands, Kosovo, supporting awful regimes as long as they're anti-American etc).
Inevitably this means he's had to take allies where he can get them. And equally inevitably many of those allies are pretty eccentric conspiracists. Because of the nature of the causes he's backed some have been antisemitic.
The mural isn't a one off example. It's happened time and time again and been reported. To show he really understood the problem he'd have to start by acknowledging this. But I don't hold out much hope.
It's part of his political base. Not a major one perhaps, but a section. And he is a politician who never challenges his base. There is a structural contradiction between Corbyn leadership and the ambition to deal with antisemitism in Labour.