You're trying to split hairs over semantics. If you take a blade of a knife( not a fork since that's not what we're talking about) and push it straight into something it is still cutting that object, it's not slashing it, but it's still a cut. The way you cut meat isn't the same as cutting a person it's more sawing then slashing but it's still cutting, there's more than one way to cut something and a stab still cutsas the blade enters the object.Buggy is immune to being cut. A stab is a pierce. There's a difference; you see this difference every time you jab your fork into a piece of food, then cut it with a knife.
This is also the definition of the bara bara:
"The fruit's major strength, as demonstrated by Buggy, is that the user becomes permanently immune to damage from being cut or pierced, regardless in what manner, be it vertical, horizontal, or otherwise."