It's a myth for sure. Most people don't take into account that a pitcher usually sees a slow decrease in velocity in the time leading up to the surgery, and therefore come back throwing harder than that. But comparing healthy velocity to hurt velocity is not evidence of anything of course.
However, I have read (and I can't remember where now...) an article or two that did show some pitchers throwing harder after the surgery than their healthier earlier career speeds. Thing is, the surgery was not the reason for the increases, but rather the more focused throwing, strengthening and stretching exercises that the pitchers did during their recovery, and continued to do after getting back to their full strength. Basically, they never knew they weren't actually at their full strength before, but maybe could have been if they'd been doing the same exercises before the surgery. And the difference was like .7 -1.2 mph or something.