They can’t make ANY argument. They can’t say if only Avrohom were our father. If only we were the Jewish People, if only we were all prophets.
But what they can argue apparently, is that had they a prophet, of their nation, as great as the prophet Moshe, taken from the Jewish Nation, things would have been different. They didn’t argue that had Moshe been their prophet things would have been different.
I know you love the art of debating wolf, but I really don’t. I don’t think I can explain it any clearer. If you still don’t understand what I am trying to say, maybe someone else can answer your points.