Is the divorce due to the therapy, or is the divorce the result of the issues they went into therapy for and the resolution was the decision that divorce was the only solution to irreconcilable problems?

My therapist always said he didn't advocate for or against divorce; that wasn't his job. Our future was what we needed to decide for ourselves.

In our case, our therapist knew we were committed to our marriage, and thus he worked with us toward what needed if we were going to have a chance of that actually happening. We didn't feel our problems were so severe to be unresolvable, so that was our goal. If we had not been able to work together towards those ends, we may have ended up divorced, but that would have been our decision based on our inability to work through our relationship issues.

Other people go into marriage counseling with a marriage in such disarray and with such severe issues, that they sort of go in already realizing the odds of things working out and saving the marriage are slim at best. Therapy is a last ditch effort and they realize things are far too gone. Perhaps one or both parties aren't completely committed to even making the effort in therapy. Sometimes things are so dysfunctional by that point that there just is no going back.

Other people go into marriage counseling already having decided on separation; they may have children involved and want therapy to get through the inevitable in such a way as to be least traumatic for all involved and the therapy is almost a mediation.

I don't think marriage therapy "working" necessarily means the couple will live happily ever after for the rest of their lives. "Working" may be facing the reality that life separate is healthier for all involved.