People love to take "extremes" and present them as the general rule. Boa sisters had weak haki (at the time of the battle) so Luffy overpowering them with gear 2 and 3 makes sense, it's in the "realms of possibilities" unlike some ludicrous and trumpet up takes. I don't know what you're guys debating, and I don't have a horse in that discussion, but haki isn't just a tool, it's everything. How many top tiers you know without haki? None. How many top tiers you know only with haki? At least three.
But they are for sure using Haki (BM, Kid, Law), but it's the invisible one since their vs is more about DF (Big Mom's giant mode, Kid and Law's awakening) so Oda is focusing on showing this instead.
We can assume Big Mom was even using AdCoC, but "invisible", or offpanel, since author is pretty limited when fights.
Anyway, what is for sure is that Big Mom was going all out, that's why she spent a year of lifespan to powerup. So the excuse of no AdCoC on panel is irrelevant, because her doing that means what she had before that wasn't enough to beat these 2, and that includes AdCoC.