They probably aren't taking him down with anything less than high diff. They can't touch him, and will be taking a beating the entire time. They will have to outlast him.
That's giving them the benefit of the doubt, based on current feats he can solo them. But I'm banking on the combination of their haki + natural durability being good enough to hold out in a 2v1.
Smothering them probably won't work, Queen can use laser blasts to blow himself out of there, King can fly and keep Katakuri busy anyway.
They're getting thrown around by Marco, Katakuri can replicate everything Marco has shown if not more in his own way. Sneak attacks won't work thanks to FS + intangibility (consider this akin to Marco's regen on defense), and he has instant spawning range which even G4 with its mobility could not avoid (for offense we saw Marco using ranged attacks to pressure them at the same time). Just like in the Marco scenario it would end up being a battle of stamina, can't justify anything lower than a high diff win for the duo atm.