PK is 7.62x54R, not 7.62x39, and it's NOT based on AK. You are talking about RPK, but that one is magazine-fed.
I think it's probably safe to say they were carrying the RPK on those little ponies,and that is what they meant when they stated "PK". It may not be technically correct,but I've heard the RPK refered to as a "PK" numerous times.
To be honest,I think the 762x54 PK is more correctly refered to as a "medium machine gun" than a "light machine gun".