Nietzsche doesn’t “speculate” that slave morality kicked off with the Jews because they were a particularly oppressed group, and really got going under Christianity. He states it outright, and he doesn’t care whether any oppressed group could have done the same. He interprets the known history of Christian and Jewish morality as being the history of “slave morality” and calls it “genealogy” - it isn’t an economic argument.
That’s all I’ve got, I don’t care.
Just the very starkest most ratiocinationary combination of a contrarian red herring and a false dilemma