Perhaps by simply being stated as one of the strongest alongside with Zoro, with more authority, higher bounty and obviously by actually fighting and beating stronger opponents?
If Sanji gets all of types of Adv Haki, and fights someone below Yamato's opponent in strength, does it still means Sanji is superior because he has every single damn advanced haki out there?
It is such a simple logic, doesn't even matters what he might get as power ups, he just needs more AP by default, just like Zoro