I do not believe this is true. Some people come back stronger than ever but it's not that often, let alone routine, that you see people come back with MORE velocity than they typically did.
I have seen a lot of guys come back from the surgery and throw harder. From what I have read/heard from different people who have had it or know people who have had it, the reason behind it is the rehab and not the surgery. That would explain what you were saying Yoda about shoulder = velocity. All of the intense rehab you have to do to get your arm back affects the amount you are lifting and strengthening your entire arm and not just the elbow
From what I understand its a combination of a couple things, mainly an increased focus on arm strength during rehab. Also, you take a guy who is throwing 150-300 pitches a week during the season (warm ups, practice, games) and suddenly stop letting him pitch for 9 months and that arm get more time off then its probably had since he was 12.