He's an old school left wing politician who has spent his whole career outside the political bubble, protesting the awfulness of the political consensus.
He thus generally assumes that the USA and the UK are up to no good (frequently with good reason), and his fellow travellers are the fringe of people who feel likewise.
Which includes more oddballs, conspiracy theorists, and racists than the mainstream is used to. And he's used to mutual support between his fellow travellers, and so hasn't been as quick to condemn things as he probably should be.

I don't think he's antisemitic at all.