The author doesn’t assume that there’s no problem that takes longer to clear up than the 60 studied. She only needs to assume that if those problems exist, most people do not have those longer-to-clear-up problems.

The author assumed that most people have one of the 45 problems that cleared up within 50 weeks. But this overlooks that most people might have one of the 15 problems that took longer to clear up.

Even if you think having multiple problems could require more than 50 weeks to clear up, the author doesn’t have to assume “no one” has more than one problem. She would only need to assume that most people don’t have more than one problem.

We know that certain problems cleared up with weekly behavioral therapy sessions. So, there’s evidence that behavioral therapy works. Also, the author doesn’t conclude that any kind of therapy will be enough for most people; she specifies behavioral therapy.

The study concerns the time required for various psychological problems to clear up. Whether the sample had more problems than average is irrelevant, because we can still measure the clear-up rate of the problems present in the sample.