it is not on him to prove manipulation wrong.

It is on the manipulator to give data that what he or she is doing actually helps - controlled study data. To proceed with treatment without controlled study data that shows the treatment helps is mysticism, not science or medicine.

An osteopath should only embrace that philosophy if there is a proven scientific reason to do so.

Uncontrolled "I do this and it helps" data is subject to a very strong placebo effect, as well as confirmation bias. The medical record is littered with useless treatments that persisted for years because they seemed to make sense. And often people continued to believe in the treatments even after they were debunked because the biases were so strong. See prostate massage for chronic prostatitis.

Note that I did not accuse you of doing these things, you may have controlled study data to back up everything you do. But I did not see it here, aside from a reference to "actual evidence that exists."