I think you're totally misreading this, there's no pretending to do a medical procedure. You have a therapy group and surgery group and see how each group does over at least a few years. Just because you did a study showing surgery improved mental health is not that meaningful because there's not much context to it. Giving somebody something they want will most likely have them more content than not doing it, especially in the short term (aka honeymoon period).Pretending to do medical procedures on people and then not doing them just to see what happens is an ethical nightmare and there's very many reasons we don't do that anymore.
What would a "control group" for therapy even look like? Deliberately giving bad advice? "We know you want help dealing with your gender dysphoria, but no and please don't try and help yourself"?
Or is the control group just the vast majority of people that exist that don't need therapy?