Most likely high skill. Mihawk said that a sword without grace is no good sword, and Zoro, who already was proficient in swordsmanship, asked Mihawk to teach him the „way of the sword“. So I guess it's a combination of Peak Haki flow + peak technique. I guess being a Conqueror helps too, as Conquerors have naturally higher potential and a stronger will, which tames the blade, though I guess Haoshoku coating is not directly involved.