Half right, he has no ryuo and so can't DAMAGE Kaido, but pre-udon G4 Luffy beat Katakuri and was not a weakling despite not having ryuo, it is a specialist technique. Sanji is lined up to fight king.
Basic CoA/Hardening/ Barrier Ryuo/ Penetration Ryuo
Zoro and the scabbards have Barrier Ryuo, Luffy has Penetration Ryuo, Sanji only has hardening, but he is a CoO specialist, and ryuo is only a dealbreaker against Kaido.