Alexander is maybe reading the wrong therapy books and literature. He should look up Bruce Wampold's research, for example. The dodo bird effect is well documented: therapy works relative to all sorts of placebos, but therapist effects dwarf therapy effects, not everything works for everyone, therapist-client cohesiveness is paramount, and there are really no replicable superiorities of any given therapy with regard to general functioning and well-being, even if specific therapies might be more effective for specific behavioral problems.
To be fair to Alexander, therapy research and literature has the kind of weird duality he points out, and it permeates academics as well, where the problems he mentions are maybe even worse. Everyone does know about the dodo bird effect, but then people either embrace it or they ignore it.
A big problem is the exact same thing that's permeating all of medical science, which is a huge pressure to obtain grants and notoriety for discovering a cure. There's much more rewards for overhyping than being sober in science. You could replace therapy with drugs in his essay and it would read the same. In fact, I sincerely was confused for a bit by his figure because the exact same sorts of figures have been published about psychiatric medications, and it took me a while to be sure what he was talking about.
One thing that's maybe not immediately clear to the general public is that with psychiatry and psychology there's an additional pressure to "be scientific" which then often backfires in the way he's witnessing. Both psychiatrists and psychologists are the targets of witch doctor labels, which then leads to this behavior where researchers and theorists jockey to outdo themselves in being "real scientists". The good part of this is that then you get a ton of randomized controlled studies and so forth, but the downside of this is you end up with this additional reason to promote your drug or therapy as being somehow better than treatment as usual, or to denigrate other interventions as being "unscientific". That is, certain things (e.g., CBT in therapyland, or arguably things like prozac in medicationland) become banners that you're expected to carry if you want to be part of the "real science" club. Criticizing the efficacy of these interventions implicitly doesn't just mean criticizing those particular interventions, or criticizing the specificity of their mechanisms, it means you get labeled as not being on board with the "real science" club. It is just one more reason for hype, in addition to the typical hype that is flooding biomedical research at the moment.
Alexander mentions that he feels like he's never seen "sudden, extraordinary, long-lasting" effects of therapy. I'd suggest it's one of those situations where you get to pick 2 of 3, nothing more.