I'm sure that Corbyn is genuinely horrified at the idea he could be antisemitic. And that if a far-right group attacked a synagogue or something the condemnation would be real.
The problem is that for decades he's been on the fringes of British politics, especially on foreign policy.
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.
As he has had so few allies he's felt he has to back them against opponents who've pointed out their antisemitism. And over time he's become blind to their behaviour (if he ever saw the problem).
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.
I think you are generous to him. There is a form of antisemitism - the "malevolent 'Zionist' global dimension to Imperialism' - that is embedded in some hard left ideology going back a long way ...