Using the Will of Marco translation
Sanji suggests that there are multiple ways to permanently defeat or kill the Holy Knights so not just relying on ACoC. "Ways" = This implies that there is several different weaknesses.
Chopper could be one possible example.
He was able to remove the DRs simply by slapping the giant face.
His Devil Fruit may be a counter to Imu’s powers, which could be one of the weakness sanji was talking about
If that’s the direction Oda is taking, then there’s a good chance no one else in the crew will awaken Conqueror’s Haki aside from Luffy and Zoro.