No one does and no one told us it's a devil's fruit ability either so I don't get why you're so sure it is one. Also, Law's fruit does exactly what it supposed to do, it allows the user to operate on people inside the room. Based on what we know, Gandhi's fruit gives him the ability freeze things, and freezing isn't supported to prevent cuts from happening. So we don't have any reason to think it's obviously a df ability without leaving room for other possibilities.
And why's it so hard to believe it can be attributed to skill? The whole concept of real swordmanship in OP is about being able to cut what you want to cut and not cutting what you don't want to cut. This could be us seeing that in practice.
It's either a devil fruit ability (if the Gorosei are devil fruit users) or a devil fruit-like ability that the Gorosei get, probably from Imu, either way it's the same thing.
Somehow operating on people inside a room equals your cuts not doing damage? Devil fruit abilities are nowhere near as direct as that, for example, Viola can somehow create whales from her tears even though her power is to see far distances and through things... barely even at all connected to each other.
That last part isn't what the OP is arguing though, it could be that thing Zoro mentioned back in Alabasta, but cutting and not cutting aren't the same thing as cutting so precisely that you go between atoms lmfao, that is just sheer wank from overcomplicating it by the OP when the far more likely answer is that it's just his devil fruit ability.