Yea I would like this stuff to not be Haki to be honest... It makes no sense for it to be haki. I think im with Kaneki's explanation on this issue but it's likely to be wrong with how Oda wants to tie things up quickly. It makes much more sense for BOAT to be a skill tied to Swordsmanship and that masters like Mihawk should be able to cut Diamond even without the use of Haki.
But Black blade in chapter 955 is portrayed as an even higher level than that, something that mighty Oden couldn't accomplish.
As much as I don't like, strongest of swordsmen are pretty much defacto Hakimen in One Piece lmao. That shit ain't gonna sit well with @Red Admiral who have been trying to force "Shanks is a Hakiman not swordsman" notion to push "Shanks > Mihawk" narrative.