This has nothing to do with Corbyn, or left wing politics.
This ridiculous letter exposes the lie at the centre of BLM. The fallacy that racism is a simple matter of the struggle between oppressed blacks against oppressor whites, hence white guilt, and everything else is just gradations of oppression, right down to the nastiness shown to redheads.
This argument is not just naive and stupid, it's very dangerous, taken to its conclusion two extremes collide. In the United States it is not unknown for blacks to express hostility towards Jews because of the belief that Jews are the chief white oppressor, what with secretly running the world and what not. Which bizarrely dove tails with white supremacists chanting "Jews will not replace us", coz they know Jews are out to destroy the white world order, that would be the same white world order some blacks believe is run by Jews! Coz every true Aryan knows the "tribe" is out to dilute all those pure white bloodlines.
Jews are caught between a rock and a hard place. In such circumstances, given the terrible consequences of complacency, is it any wonder that establishment Jewish groups trashed Corbyn? Even hounding "unacceptable" Jews who dissented! Many Jewish Groups and publications behaved appallingly during the Corbyn era, they lied and dissembled and you're still doing it now, but if I were in your situation I'd probably do the same.