Isn't the name of the attack already an indicator that he's using fire? Hiryuu. As far as my understanding of Japanese goes (which granted, it isn't high at all so I could very well be wrong), Hi means "fire" while "Ryu" means dragon, so the literal translation of this part would be fire dragon. Just like "Hiken" means fire fist. I think it's a combination of both fire and haki, which I think makes sense. IMO, it looks too bright to be just CoA, but some parts also look too dark for it to be just fire.
Fire imbued with overflowing haki would be my guess as of now. As to why he would be able to suddenly summon fire when he has never done this before, I think he used the fire he cut from Kaido's Boro Breath to potentiate his attack or create a new technique.