Remember when Zoro learned to cut steel in Alabasta and he heard the breath in all things? Was this swordsmanship or was this his first encounter with Haki?
It's not Haki, but it's similar to Haki. When he heard the rock falling, he did not dodge them, but instead he knew where the rock were gonna fall. Kenbunshoku Haki has something similar, but the user predicts an opponent's moves which makes it much easier for them to evade the attack given enough skill. Zorro was also able to sense where his sword was, while no Haki users are known to have this ability.
Zorro didn't move and he said he was not evading those rock
Haki users can evade attacks.
Though the power allows the user to predict most attacks, it can be circumvented by various means. It cannot predict inherently random attacks; for example, Luffy managed to bypass Enel's Mantra by bouncing his fists off a nearby wall, to prevent himself and thus Enel from knowing where they would land. (source)
So it was clear that Haki users can't use their Haki power to predict a movement that was not intended. Therefore, Zorro's ability to sense those rocks was not Haki.
Another proof that Zorro couldn't use Haki at that time is that he was able to cut through Daz Bone's metal (which is actually possible with enough skill), but he couldn't cut through Enel's lighting (impossible without using Busoshoku Haki).
The idea that makes sense to me is that; Zorro is using both kinds of Haki in tandem.
One, he can sense his Iwashimizu sword which in itself is special as due to its rarity and nature, + the swords in One Piece have unexplored depths e.g. black swords, and cursed swords. Hence, the ability to sense it with Haki as it is probably imbued with another's spirit.
Two, knowing the breath of things as it is stated, maybe how Zorro utilises his Haki, in this event he knows the breath of steel, so he strengthens his own steel with Haki just enough to cut Das Bonez.