It is basically just a mummy-burger. Drying foods to preserve them is a technique almost as old as humans themselves. Notice how every time you see someone talking about this experiment, they used a burger with no condiments? The condiments are too moist, they would provide a foothold for decay.
Mist that burger down every day with a spray bottle and I'd put a great deal of money on it decaying just as fast as you would normally expect food to decay.
Spoiler: the author makes a completely homemade burger and has it dry out in the same way as the McDonald's burger, thus debunking the theory that the longevity of the burger is evidence of foul play.
