Did those samples contain xenoliths?
Yes. See the links in my previous message.
And why do I find your claim that this "just happened" rather suspect,
He claimed that he had just
heard which is something quite different.
since K-Ar dating hasn't been widely used since the 70's?
Depends on what you mean by "widely". It's still used, maybe on the order of 10-15% of all dates. It's low-cost, easy, well-understood, and known to be reliable almost all the time. Often K-Ar dates are cross-checked with other methods to eliminate the possibility of excess argon.
Did they do 40Ar/39Ar dating to eliminate test errors from nonradiogenic argon? If not, why not?
Surely you jest. Ar-Ar wouldn't give them the answer they wanted. But you knew that.
Since that's the most commonly used procedure for dating rocks these days?
The most commonly used procedure these days, over 50% of the cases, is U-Pb concordia-discordia dating, for several reasons. Ar-Ar is widely used but may suffer a decline in usage; new regulations for handling irradiated material post-9/11 make it more cumbersome and expensive.