They aren't equal.
And Marco will beat king.
And Marco will beat king.
- Marco fought King+Queen for roughly an hour while protecting samurais from Queen's virus, weakening him, and that after an off-panel fight with Big Mom
- After holding Queen for Chopper to fight, potentially left this fight to him to handle King in an off-panel battle, maybe with Perospero shooting arrows on his back from the distance
These moments most likely happened to increase the tension of the King vs Marco battle.
Just imagine if Marco beats him still despite all these advantages