FS and koka are completely incomparable
FS is an advanced technique, koka isn't. People who have FS are literally CoO masters.
it wouldn't be proof of contrary. Rather it would simply mean that that character could use flow and chose to flow some of their haki onto their clothes as well to enhance their defense, in a similar way to how swordsman flow haki to their sword to enhance their offense.
We have blatant evidence of characters using koka on their body but not their clothes. Hence it is possible that Sanji could have koka and that we just don't see it.
My reasoning as to why that is likely, however is that Sanji can block cutting attacks with his haki, whereas fishman island Luffy, who had hardening, couldn't. Feats seem to indicate that Sanji's CoA> Fishman island Luffy's.