As someone who's been watching professional Korean SC competition for almost 10 years, the strategy described by your brother can be found from many high class players. Unlike Warcraft3 or other RTS, units in SC have very low health. Even if you have 5% more units, you can easily lose a fight within seconds from bad unit control, bad fighting position or just bad luck. It's never a good idea to attack just because you have 5% more units. Good players will continue play cautiously to make sure the opponent never gets a chance to take those 5% back. In the end, they win by the accumulated bits of advantages from various battles or economy advantages. You don't always need an glorious "final battle" to win the game, as long as your advantage is big enough (say 50%), you opponent will know that he lost and leave the game.

This is probably why SC has become very popular and still remains very popular. There are constant conflicts throughout the game trying to increase or take back the small advantages.