It isn't. The problem this program solves is thwarting a naive attempt to alter the state of the USB bus. The design assumes the attacker is not aware of the consequences of adding or removing devices and has no reason to employ spoofed devices or any other Ever Greater Adversary Regression techniques you can imagine.
After they got bitten but tools like this usbkill once, ID spoofing will just become the standard practice, and it will be made so easy to do they don't even need to think.
