There was a substantial subset of the fandom that were convinced beyond all reasonable doubt that Sanji would peak at the level of King or Marco, so it's understandably jarring for them to come to terms with the possibility of Sanji fighting a Gorosei. Funnily enough, the reason said people thought that's the level he would peak at is precisely the reason they don't believe Sanji could fight an Admiral or Gorosei; power scaling over narrative, this was their mistake. Oda will justify Sanji fighting and beating anyone regardless of established power scaling because the narrative of the story says so.
This narrative being that Sanji is a pillar character whose purpose is to stand alongside Luffy as his Wing at the top of the mountain and fight off all those that try to get in his captains way. Which is why Zoro and Sanji connected their victory against King and Queen to Luffy becoming the pirate king and not them reaching a new personal power milestone. Who will stand in the way of Luffy getting to Imu and defeating him?.....that is who Sanji will fight, not because of power scaling, but narrative.