He already showed that, he has haki, unless you think Sanji has done everything without haki, which is dumb as hell
Or that the Okiku pierced Kaido's hand without using haki, and that other Scabbards even more without haki
Oda is just inconsistent, he praises aesthetics, unless there is a point on showing something, like a during a new move
The fact that Sanji's CoA is being highlighted now is because he mixed it with his flames and created something completely new, thats the point here.
Sanji already said he can use haki on his any part of his body, and Zoro did the same against King, but it wasn't shown