Of course he is not saying they will be supported by maaser rishon. Maaser rishon only goes to real leviim. But he is saying they are exempt from taxes, the army, and all sharing of societal burdens.
And of course he is saying that we are required to support them. Precisely the way we are required to support leviim–which obviously goes beyond just giving them maaser rishon if that is insufficient.
As far as acharonim, I meant you should look at how acharonim who are saying pshat in the rambam answer this contradiction; not at acharonim who are saying other opinions.