I guess I do not understand the "go both ways" argument. If the whole point is fairness and equity, then why do something the allows white performers to hang on to the same advantages non-traditional casting was supposed to diminish?
But this comment was not even about nontraditional casting, since this is not that. It was meant to point out how silly the idea that black Jews are such unicorns that you cannot present them credibly on stage.
And even if the OP truly believes that black Jews are so rare--so what? He can accept people singing instead of speaking, but skin color is where he draws the line? If someone chooses to accept the convention of singing in a musical, they can choose to accept a casting convention as well.
You have to wonder why people choose to fight that theatrical convention and not others. Not a peep about a blackout at the end of a scene, two-dimensional scenery, vocal projection or any other number of conventions---only this one.