The much vaunted reliability of the AK is a myth. The claims on unreliability in the Eugene Stoner design are only valid if you don’t lubricate the weapon.
The Stoner platform has been in service for over half a century now because in that time no one has been able to produce anything better.
“...poured over every inch of its mechanical parts...”
I doubt it. I suspect that the bolt itself, and the various internals of the lower, were still clean.
More important, however, is how close did the rifle get to a malfunction by the end of that test? If they poured mud on it, fired it 5 times, and it worked, great. But if they left that mud on the weapon, dragged it through the woods for 30 minutes, then tried the test again, would it still pass?
I’ve carried the M-16, as I am sure have many others on this board (and most in more demanding circumstances than I), and while most any weapon will probably function for a short time after being fouled like that, the AR design really doesn’t like to stay dirty. Yes, every part is fitted very closely to prevent the ingress of contamination, but by the same token, a lot of those closely fitted pieces don’t work very well when they are contaminated. The tolerances around the splines on the bolt are very close; if dirt gets in there, the bolt won’t properly lock into the chamber, which in turn can cause wear or stress on the bolt, accelerating the possibility of cracking the bolt face or damaging the splines. You also have the very small components of the ejector that can very easily become jammed if something gets into it. I’m not a gunsmith, but it’s pretty easy to see that the AR design is not going to run well if it’s full of FOD.