Impact debris...hmmm.
An old cometary impact, assuming the comet was mostly volatiles, would not leave behind anything except the lunar dirt spectra after millions of years. A solid object impact would leave at least some ejecta behind that was characteristic of that solid object.
I'm no expert, but it seems that the spectra of ejecta would have a signature of lunar material only for a comet-type object, lunar material plus asteroid for an asteroid-type object.
That's assuming you can distinguish between lunar and asteroidal materials lying on the surface with a spectroscopic instrument. I don't know if that's true or not.