That doesn't change that those conditions alter metabolism. It doesn't mean it's not possible to eat at a deficit, but it becomes much more difficult to do so without becoming malnourished and takes special planning and careful monitoring. You can be obese and malnourished if you aren't receiving enough of the right nutrients, so getting the help of a registered dietitian is very important in those situations if someone is struggling with weight loss and needs to go on a very low calorie diet.

I think we also put too much pressure on massive weight loss and reaching an ideal weight or goal weight. Even a 5% decrease in weight is clinically relevant, and losing 10-15 pounds is a lot simpler than losing 50-100. Someone whose weight ballooned because of an undiagnosed metabolic disorder will not only have more physical trouble losing weight than someone else, but they have a discouraging distance to go, especially with all the people who act like all levels of obesity are created equal. They aren't, and that manageable 15 pound weight loss is worth going for even if it takes as long for you as it takes a metabolically "normal" person to lose 50 pounds. 

This isn't a criticism of anyone in particular, just of the general culture around weight loss where the only success stories that are publicized say something like, "I lost 3 pounds a week for a year and now I'm literally half the size I was!" For someone who struggles to lose half a pound a week in a healthy way, that's more discouraging than motivating. I think I'd like to see more in-progress publicity, where someone says, "I only lost 10 pounds, but we were able to decrease my blood pressure medication!" "I've lost 15 pounds and my knees already feel better!" Basically just more realistic weight loss expectations for people with those disorders and diseases rather than shaming them because their bodies react more slowly to changes in energy balance than other people's.