After reading this article (without reading the actual study), this is not surprising. It seems like their measure of what constitutes treatment-resistant depression is a failure of two or more antidepressant trials, but it's widely known that depression has multiple etiologies, only one of which is possibly addressed by an SSRI.
For example, they cite right here in the article the comorbidity between personality disorders and treatment-resistant depression. People who have personality characteristics that include e.g. difficulty with emotional regulation, a proclivity for tumultuous relationships, or self-injurious behaviors frequently find themselves struggling with employment, relationships, and more. This makes people feel depressed, and there is no pill that's going to help.
I'd be interested to know the level of comorbidity with trauma history, as well.
Study participants and clinicians offered valuable suggestions for improvement. These included...offering more diverse psychological treatments beyond cognitive-behavioral therapy...
This is clutch. CBT is not going to work for the group described above. People need competently delivered Dialectical Behavior Therapy to learn adaptive skills around emotional regulation, distress tolerance, and interpersonal communication, and Cognitive Processing Therapy for those with trauma-related anxiety symptoms and who are ready for it.