You means the same way that kizaru isn't?
Or do you mean the same way in which rob Lucci also wasn't using armament when he and luffy were fighting the seraphim.
And that's also cap. We know he is using it against zoro since he used it while in base to block zoro when they fight first popped off.
Canonocally though we have yet to see sanji using amrmamt haki hardening where his limbs turn black upon using it