-Yes there is aura around his fist:
-Because it's the initial fist and in chapter 1001 we saw a warm up round (no Haki from Kidd, Zoro, Killer, no special technique from Law, no Adv CoA from Luffy).
And there is no statement of any kind saying that Kaido had his brain damaged or something like that, which will indicate us that it was Internal Haki.
Luffy damaged him since he has more AP now thanks to Barrier Haki, that's all.