Could it be Hardening? Yes
But in a Chapter where there are dozens of Mistakes that seem Intentional to add to the Place's Mystery,
I don't think We can consider that Panel as Proof
Sanji's Hardening wasn't just never shown, there is actually no Feat that He would have failed to do if He didn't have Hardening.
And then Oda gave him Exoskeleton, and it doesn't just boost his Durability, He can recover from insane Injuries
I'd also rather believe that Vergo cracked Sanji's Leg cuz He didn't have Hardening,
I mean if his Hardening can't even Protect him from another guy's Hardening (Who is nothing compared to Top Tiers), then it's useless to have it, and an unnecessary de-hype, it's better that Sanji simply doesn't have it, not that it was that Weak
Especially when his Fire allows him to deal Damage to anyone, regardless if his Kick was Strong or not
And add Exoskeleton now, it's clear that Sanji is simply not the kind who uses Hardening