I have noticed this kind of thing. I've done therapy on and off (sometimes reactively, like after my Dad died; sometimes a little more pro-actively, like when I had a brutal case of writer's block during my dissertation prospectus). Never anything I would rate as very serious or urgent. I would say the therapy was not un-useful, in the sense that I didn't mind taking time out of my day to talk to someone. But I also basically felt like the eventual remediation mostly just stemmed from the acute phase passing or steps I took outside therapy. The thing that got me over my writer's block was mostly writing. The thing that helped me deal with my Dad dying was mostly more time passing and my life moving on.
Meanwhile, I have noticed that a lot of people in my life with higher distress loads, and particularly people like the author of this piece who report suffering from fairly serious mental illness, invest a lot of resources into self-help. I think that's a good instinct, to say "I have a problem and this is a tool that will help fix it", analogous to seeking medical care for a physical ailment. But my experience is that the vast majority of those people seem to make little apparent progress via self-help, and instead typically continue to report being extremely distressed (though, punctuated with confessionals about how much better things are). For most people levels of distress in life seem to me to be primarily associated with lifelong, or early socialized dispositions; or with major material facts of life (i.e. single/married, broke/affluent, healthy/sick, undergoing a major life change/being severely stuck/doing fine) rather than the particular steps they take to manage or mitigate it. That doesn't mean therapy doesn't work, because maybe the counterfactual is they would have gotten much worse absent therapy.
A few friends from when I was a teenager, people who really struggled with severely gnarly upbringings, abuse, broken homes, early life drug dependency, etc. even went into becoming therapists or social workers. And the strange thing is, it externally seems to me like they're about as troubled as they were 20+ years ago, modulo having a little more money.
I guess I would distinguish a lot of these cases from people who are acutely suicidal, people who are actively seeking drug rehabilitation, etc. There seems to be at least some evidence for intervention in those cases. But I am talking about the kind of cases I think you're talking about.
So to your broader point, despite again the fact that it's expressed a bit stridently, I think there's something there: the evangelism about self-help's benefits often seems to outstrip self-help's apparent benefits. There's probably some element where part of therapy is basically tricking your brain, so the apparent sentiment of someone who is clearly having a hard time beaming about how much better they're doing might be part of the plan anyway. But externally it can feel jarring to hear "I'm really turning a corner!!!" from someone who's been struggling for years and continues to struggle.