Actually I believe he was, against the Soviets or their puppet Afghan troops, and then later perhaps against everyone else but the Taliban. But against the Soviets and/or Red Afghans for certain.
Not sure he carried that Krinkov then though.
However the Soviets themselves had switched to the 5.45x39 by then. Michail Kalishnikov never liked or approved of the change in cartridge. Of course the change from 7.62x39 to 5.45x39 is not nearly so extreme as going from 7.52 NATO to 5.56 NATO. The 7.62x39 is more akin to the 6.8mm now proposed by some for the M-16 or XM-8.
One of the biggest problems with the M-4 is that the barrel is too short to get the round completely up to speed. I imagine the Krinkov that OBL displays has the same problem.
Assuming you are right that OBL actually participated in a fire fight (which I seriously doubt) short barrel, small caliber weapons do not lend them selves well to the terrain in Afghanistan. I am speaking from the viewpoint of an 11B40 combat vet, not a military theorist or gunsmith.