Rambam also says (ibid, Hlalacha 8) that if a navi appoints someone from a different shevet to be king and he keeps the Torah he is considered the king
But he also says that Only Davidic Monarchs have a right to sit in the Azara,
or
read the Hakhel
P.s Eh.. Everyone is aware what he says in the Moreh,and that everyone else strongly ( and more)disputes it.
AND that some try to reconcile it with other conflicting ,and arguably more authoritive positions