120+ technique is simply greater than that found in rec players.
The guy doesn't even really play tennis - he just likes to do athletic stuff and show it off on youtube. A serve is basically a throwing motion..
If you have a big arm you can pick it up quick and blast some big flat serves. But clearly there are rec players who serve 100+ - even low ranking ones.
This challenged is resolved.
Rec players can serve 100+ It's not so magical mystical thing that hardly anyone can do. My gut feeling is that most men of average weight and height will hit right around 100 mph with a serve without significant technical flaws.
If you are bigger and/or taller - you should be hitting even harder serves. We know from baseball that throwing speed is correlated with these two factors..