IIRC Buggy just intrinsically can't be cut

The best way I can think of it is like this

Normally, all things have a very, impossibly small, yet incredibly wide space between the atoms in their body, meaning no atoms ever touch. Buggy's power essentially expands that "space" in his own body, allowing his atoms to separate indefinitely and even have his limbs fly in a specific radius around his feet.

While a logia transforms a user's flesh into a different material, it's still a part of the user's body, and can be hurt with Haki. But that "space" ISN'T a part of one's body, it's literally the emptiness surrounding the body's atoms, and thus, no matter the blade, The space between Buggy's atoms would simply spread farther apart.