You know what ive never really looked into this scene but i think youre right.
Because only haki ripped through the cliff not like zoros normal slashes he can do with his physical strength.
As you can see the sword was leaking ryou at the time too.
This had 0 physical strength behind it and was pure haki unlike some of zoros slashes say from dressrosa.
And thinking back to how this fits into what hyou said about haki
What happened with enma there fits more with the second explanation than the first (as does what wb and roger did)
Even the gag one with sanji
Hes sat down yet still manages to do that big a slice. Once again likely with flowing haki.