The reason the patty doesn't rot it because the patty is small enough that it loses all of its moisture quickly and the heat it is cooked at sterilizes it. Thus, the conditions for mold simply do not exist. Adding katsup or using a larger patty will give different results, as there is more moisture available.
This guy did an "expiriment" without doing control experiements and drew a conclusion without nearly enough information.
It's not about whether McDonalds is healthy or not, it is about good science vs bad science.