It would have been cool if Kidd was smart enough to coat the horns in haki and try to make them pierce her "iron balloon" skin, as piercing is way superior to blunt/area/smashing type damage for breaking through tough defences.
But this is Oda of course lol. We've seen how much he favours certain characters over others and wastes potential on some characters like Kidd.
In an arc with freaking emission and internal destruction haki manipulation being revealed and explored, we could have had Kidd utilising it and thus having a way to counter Big Mom with it. He could have learned it from Hyogoro alongside Luffy and given himself and Law a huge advantage vs her potentially.
It's crazy the scabbards didn't train it to the SHs and thus Law didn't pick it up whilst he was in Wano too.
But Luffys defeat of Kaido also ultimately came down to a giant fist smacking him into an apparent fiery magma death ironically beneath Wano, so theres that too.
There's a slim chance Kidd could be rescued and trained in the art of haki fu by the haki god himself but I really, really doubt it as much as I would love and want it too. Shanks could work as Kidd's Rayleigh and give him his training arc and humble him but I doubt it will happen sadly.
But yes, to answer the thread, I sincerely doubt he was using haki on the horns at all but he should have too. He shoud be coating his contraptions in haki, especially their weak spots but alas. Kidd is too much of a meathead. I expected amazing armanent haki mastery from Kaido and his crew, and we got his commanders and tobi roppo barely knowing what haki is, well barely using it.