The first part about the body expecting sugar, and therefore causing people to get fatter makes sense. The other “science” parts sound like the start of “educating” the public in preparation for lawsuits.
Maybe it makes sense, but is it verified by experiment that drinking diet soda (or, presumably, consuming other artificially sweetened products) correlates with weight gain? Many things that "make sense" turn out not to match up with real-world experience.
I blame Big Sugar!
The most interesting story about diets that I have heard involves the Shangri-La diet. I first read about it in Freakanomics. It was a diet devised by a guy who used himself as a lab subject and tried hundreds of different diet approaches, none of which worked without significant sacrifice. He devised a theory that says all humans have a internal thermostat that determines when your body attempts to stockpile energy or not. He figured out a way to trick this calorie retention signal and loose weight.
Lots of links there.