Another alternative explanation: general anesthesia's disassociation alone combined with an expectation of transformative psychological change results in transformative psychological change similar to ketamine's disassociation combined with such an expectation.

Ruling out the general anesthesia as having an effect because general anesthesia without expectation of depression improvement doesn't improve depression seems like it toys with a false negative in ignoring the possibility of combined factors (specifically psychological disassociation plus expectations of change).