Zoro and Sanji has never admitted they were stronger than each other so I don't know where this "Sanji knows Zoro is stronger than him" comes from.
And with their rivalry, you can't omit their strength. Who has the higher bounty or who can hunt better? Strength plays apart of that to determine who is Better. Zoro and Sanji can play off each other but I have never seen either one of them say to the other to take the lead on a situation. If they have, please show me.
I mean...what's the problem?
Whenever Luffy and Zoro are alone, neither plays 2nd fiddle. We recently saw this on the roof and it has happened in Whiskey Peak, Water 7, Fishman Island, Punk Hazard, DR and Wano.
Whenever Zoro and Sanji are alone, neither play 2nd fiddle. We recently saw this against King and Queen and it has happened in Arlong Park, Loguetown, Long Island, Enies Lobby, Thriller Bark, Return to Sabody, Fishman Island and Wano.
Whenever Luffy and Sanji are alone, neither play 2nd fiddle. We saw this in Skypia against Satori and in WCI against BM.
Whenever all 3 are together, they perform similarly with Luffy usually taking the lead. We saw this in Loguetown, Enies Lobby, Return to Sabody and Fishman Island.
If you use scenes of Zoro and Sanji together to call them equals, same applies to Luffy-Zoro and Luffy-Sanji.