username, thanks for the Reb Tzadok. But note that he too talks mainly about it in the plural (and, of course, he doesn’t use the expression nossi hador).
So when lubavichers says that their rebbe is nossi hador it is ignoring the fact that there may be others of just a high a standard. And don’t forget that Tanya, which is your prime source for all of chassidus, only mentions it in the plural.