It's pretty simple: Sanji's stats just needs to be highly better than YC1's.
He doesn't need to learn skill x/y/z to reach this and that level. Plus, his exoskeleton, as well as his IJ already gives him enough power to match other YC1. He's combining armament Haki + exoskeleton to reach this high level of fire, don't forget that. Marco will have a hard time damaging Sanji. Same as Katakuri. King is more tricky due to his fire mode but flameless King would have to match IJ Sanji which is not an easy task either.
Plus, Sanji has G4+ AP with high fire lethality in IJ, his speed was fte for Queen who even tagged Marco for a bit. But against Sanji, the former was completely invisible to Queen as if Sanji used invisibility. Again, we talk about Queen; Kaido's Nr. 2.
Even with DJ and exoskeleton, Sanji can match YC1. He's not ~WCI Sanji since wearing his RS several times awakened his Germa genes.
Honestly, Sanji is one PU away of surpassing them all, even drastically depending on the PU, either way.