I believe it's more likely.
I would suspect that to get overweight to that extent they may have some serious issues around food which simply are not addressed with diet alone...so when the diet stops, they are less well-equipped to maintain a moderate, healthy approach - it's all or nothing.
I'm going to do a slightly twattish thing now and use myself as an example.
I'm about 2.5 stone overweight, which was put on fairly slowly over the last decade, but am currently consistently losing 1-3lb a week. I've cut down takeaways (still have one a week - but a much smaller one!), upped my portions of vegetables which naturally reduces portion size of the rest of the meal and began going for walks in my lunch hour and generally taking more opportunities to be active. So, with a few tweaks, it isn't a world away from what I was doing before. I believe that provided I keep an eye on things I could eat this way indefinitely.
Now take someone who was previously eating 3,500 - 4,000 calories a day. They've got a lot to lose so want to see bigger losses. Rather than tweaked, their diet is totally overhauled - down to 800 cals a day, when realistically at their size they could comfortably afford to eat twice or even three times that amount and would still be losing weight. They're in the gym two hours a day religiously, maybe with a pt who is pushing them hard. They know, on paper, what caused them to put the weight on - but at the end of it, they still have no idea what a healthy, sustainable diet looks like - because for most people, what they are doing when dieting is not sustainable.