This is seriously stupid. Until your mid twenties even, the body is either growing or in some way maturing. Stress training helps to shape that and even transform it. Excess training is beneficial but only if it is applied to all aspects of the body.
Unbalanced strength is clearly unwise. It obviously sets you up for a serious injury.
Perfecting skills for a specific sport is done easiest when the body is already in overall peak shape with even moderate skills to begin with.
We really have to lay off these kids and focus on skills. Back in the day, strength took a back seat far too often when it actually mattered. this is the reverse..