I think everyone has hardening when literal Maine fodders have shown it. The anime even showed Sanji using hardening and even though expanding the fight is alright, I don't think they will actually add an ability that someone doesn't have. Like, I don't think they can just show Zoro using FS like that. I think people like Sanji, Marco and Kidd do have hardening for sure but Oda is just a little inconsistent. Flames are a big reason for Sanji and Marco at least.
The point is, when Zoro used CoA hardening for the first time in the Pica fight it was made out to be a really big jump in terms of offensive power. Same thing with Luffy. Same for Sai as well - he was only proclaimed as a continent splitter once he unlocked CoA hardening.
And it's not like Hardening can only be used offensively right? Defensively it's useful as well to block stuff by applying it to certain areas - I certainly think Kid, Marco and Sanji are using CoA, just not hardening. Maybe it's because they never needed to master that ability in the first place Idk.