No, I am joking about the Holiday Inn, but the limit of my geology knowledge is one class that I took for fun a couple of years ago. I did, however, have a good grasp for the concepts. I hope though that if I accidentally mislead you that there will be a real geologist along to smack me upside the head.
Anyway, if I follow the story correctly, the scientists are not claiming that the dated rocks were those that impacted the moon (as you said, that would be foolish). I believe they are following the theory that the moon "seas" were created by collisions big enough to create a break in the moons crust through which magma flowed and then solidified, creating newer, darker rock on the surface. Since the moon mission that gathered the rocks landed in one of these areas, that would be the type of rocks they would be collecting. Also, since there is apparently no plate tectonic activity on the moon, the only way for new rock to be formed would be through these collisions. Note that according to the article, other scientists make a point that the rock samples were not collected from enough sites to warrant a generalization of there having been one huge cataclysmic event.
Back to the isotopes (and keep in mind that my example isotope, though a common one, may not have been the one the scientists were measuring). Potassium is a very common element in igneous rocks (a common example is potassium feldspar - the creamy pink mineral found in some granite). Argon, on the other hand is a noble gas, rarely forming any compounds. So, I am thinking that most of any argon in the magma at the time of rock formation would escaped as gas. So, the argon found in the rock samples would have been created by the radioactive decay of the K-40 after the rock had solidified and would be trapped in the rock structure.
That is about all I can come up with. Hopefully this helps somewhat. If not, I hope a geologist will wander by and enlighten us.
Ah ha! That was the piece of information I was missing. Being a gas, one can assume a static date based on the idea that the gas would not be in the rock at the time it was formed.
This doesn't help date when a rock (or planet) later was broken up into smaller rocks, but it sure looks like it could be used to date the original formation.
Thanks for the answers and the patience.