This just isn't proven at all. And likely untrue. There is a reason the dodo bird effect is controversial.

1. Most therapies have not received the number of RCTs as CBT so it's not possible to make statements like the one you made.

2. There is wildly varying quality in therapists and the quality of the therapeutic relationship is widely accepted to be centrally important to treatment outcomes. It is thus much more likely that a good therapist with a shitty tool is better than a bad therapist with a good tool. Averaged out, this would explain the same effect.