Also, some people are overweight due to, e.g., medical issues and the consumption of medication needed to deal with said medical issues.
Would leangains.com help them too?
While I study biochemistry in school, I will not pretend to be able to give medical advice, so I can't answer your question with regards to medication. However, for those overweight folks who just haven't been tending to their diet and exercise as much as they'd like, leangains and a good strength-training program is a fantastic way to shed fat and become more "fit." Although, if you're really interested in getting involved, I personally believe http://rippedbody.jp to be a better resource for the information on leangains (although leangains.com is the original source), simply because it's better organized and more straightforward.
A friend of mine has Hypothyroidism and he attributes gaining nearly 100 lbs to that because his doctor told him that might be the effect of it beeing not treated for so long.
I find that pretty silly. It may add some water weight and another 10 lbs due to that and skew the balance between lean body mass and fat. But the body has to follow the laws of physics and adding 100 lbs of mass in fat requires massive amounts of calories above your daily expenditure for extended periods of time.