untrue, when mounting from the bottom the weight distribution is on 4 screws that are being use upside down so the only thing holding the weight is the teeth of the screw and over time with use they will work their way out of the wood. will that be over 2 years or 10 years i can't tell you as it depends on the type of wood, humidity in the house, etc.
when mounting from the top the woods strength is not disrupted at all by cutting a hole in it. the shelf distributes the weight much better and the screws hold the joystick box secure from being pulled up or twisted.
if you don't believe me then try a test, cut a square hole out of a piece of wood, screw another piece of wood that covers the hole from the underside, then flip it over and beat it with a hammer and come back here and tell me it's properly supported.
it's easier to mount it from the bottom and that's why "most people mount from the bottom" however it's not properly supported and over time it will not hold as well as a top mounted joystick.