Whether it's shading or not;
A) it's not Fullbody Haki as his face is not covered. So this simply becomes a thread about Sanji using hardening on part of his body, aka literally one of the most basic functions of CoA.
B) Fullbody Haki is nothing impressive regardless. It's literally been cut right through every time we've seen it. Even Pica could use it, and he was garbo. I don't see any meaningful discussion to be had here.