And how else could he transform into a demon without Haki? The fact that Kaku mentioned his aura is why it makes sense. Haoshoku is all about your own willpower and imposing it on your enemy. Asura is the culmination of all of Zoro's willpower projected through Haoshoku Haki, which Kaido immediately took note of. It's no different than Kaku noticing the aura of that attack being different from the rest.