> In other words, finding differences in effectiveness of different intervention approaches must mean that either some of these interventions are better than placebo, or that all (except maybe the best one) are measurably worse than performing no surgery at all.

That doesn't tell you anything useful about them vs. placebo though? And "all (except maybe the best one)" seems to be entirely gut instinct not logic. You don't need to even try the surgeries to know that either all, some, or none will be better than placebo.

If both types of surgery perform the same, they could both be better, both be worse than, or both be the same as placebo. If one type is better than the other then maybe they're both better than placebo in different amounts, maybe both worse in different amounts, maybe one better the other worse than placebo, maybe one is same as placebo and other better, or one the same and other worse.

Knowing that two surgical approaches give different results gives no hint as to where placebo would perform in comparison, just that it can't be the same as both of them. It can still be better than both, or worse than both, with no reason to suspect one more than the other without other priors or actually testing vs. placebo.