Queen has a brachiosaurus devil fruit. His reach is already far and he's huge, he probably wouldn't even be able to use it while in full Zoan anyway. I don't think he needs to be able to fly in order to be a strong opponent, just because weak dudes know Sky Walk doesn't mean Queen has to have learned it just for the sake of fighting in the air. Aerial combat is Sanji's thing, even if he doesn't fight King in the air, he'll just use that to his advantage against Queen. It's like if Zoro fights King, Zoro can't fly but he'd still be a tough opponent.