Tbf, it's not really a fair comparison to make while Zoro and Sanji's fights are obviously being addressed before the Yonko. We haven't gotten an update on Luffy's fight since 1026. Zoro and Sanji both have had 3 separate chapters of scenes with battle panels in this time frame
I mean I don't think King is on par with the Emperors, but he's treating Zoro in a way several top tiers could not.
King is the first person to straight up overpower Zoro's guard (he fucking held back Hakai for a bit, so even two Emperors failed to accomplish that), and he took it a step further by overpowering a named Santoryu technique from Zoro headon, and sending Zoro flying back a considerable distance.
He basically did to Zoro what Kong Gun did to Doflamingo, only that he overpowered Ultra Tiger Hunt to do so.
For whatever reason, Oda chose not to grant the Emperors this privilege. It's weird but it is what it is.
Or consider this, Tempura Udon is above Ikkoku Sovereignty:
@Sentinel,
@Tyki_Mikk,
@Dark Admiral.