The doctor meant superficial as in a slash, not a stab or puncture wound. It does not gauge the depth of the cut per se. It merely suggests the method, and describes a wound with greater damage to the skin than internal tissue. Scar size has nothing to do with the matter. You could be covered head to toe in burn scars, and the damage could still be classed as superficial. Alternatively, you could have nothing more than a bruise on the outside, and great damage internally. It's nothing more than a standard way to describe the type of wound you inflicted.
Either way, why does it matter? This is a very minor detail to dwell on. I hope you don't feel he was minimising your wounds. It's just doctor talk. :3