That fry comparison is pretty weak. Is anyone surprised that a raw potato slice would decompose while one fried in oil would not? Ever seen mold in your vegetable oil at home? Certainly not suggesting that McD's doesn't add preservatives to their food (I just assume they do) or that the longevity of their fries is not at least a little disturbing, I just wonder why that case can't be made without resorting to such a cheap trick.
As for the burger, I've seen enough other vids/pics making the same point that I don't doubt the claim of its age, but I wonder why nobody's ever bothered to do this experiment right: Raw meat in one jar, cooked meat in another, McDonald's in a third. Compare the results at regular intervals. Because honestly, I have no idea what would happen to a burger I cooked on the grill myself vs. one from a fast food joint. I do know that I've found forgotten meat in my fridge before that had no mold and was probably months old; it was just hard and dry. I draw no conclusions from that, especially since the addition of refrigeration seriously alters the experiment -- it's just an observation.