well i knew you were going to bind it up but still the point is even in their previous encounter when Daifuku was using named attacks.. sanji was still using invisible ARNAMENT
and in that ship fight with daifuku its inconsistent to whether he used hardening.. or just momentarily and then decided that Daifuku is not worth hardening his leg
you can clearly see sanji's boots are white and the impact shock after he deflected Daifuku's blade