As you suggested, I know Ngannou is one of the very few guys in MMA with proper KO power, there's no doubt about that. And yeah Pillowfists McGregory's "power" looked like an absolute joke in a boxing ring. He only looked powerful picking off enraged manlets, with no chin or defense.
Also, I know Joyce said that, but I'm not entirely convinced Ngannou does hit harder than Fury. He can definitely punch by MMA standards, but when the fight happens, I won't be surprised at all if Fury is the bigger puncher.
Ngannou seems like someone who would throw bombs non stop in sparring, as that's basically all he knows. Fury mgiht have just been fucking around.