Agreed that Gorbachev's statement is not "proof" as such - it is weak evidence, but all that is available. Case study evidence is also evidence, even if weak.

There's a difference between poltiical writing and independent case studies.

Even if Gorbachev did honestly believe the techniques worked, he might have gotten political power because he believed them to be good in an enviroment where most people considered them ineffective. He also doesn't had a good opportunity to compare.