The former was because those apes without qi were too weak and too few in number. The latter was because Ji Xinghe had the help of Star Moon during the challenge; in fact, Ji Xinghe did not destroy many of the Empire mechas.
Upon using it, Ji Xinghe immediately noticed the difference from when he practised with qi before.
Beyond the effects of qi, the most crucial point was: how is qi restored after it has been used?
Just like physical strength, qi needs to be restored through rest and eating.
The key lies in eating; rest is a supportive measure, or rather, it is the process that allows the body to absorb food, water, and oxygen, which also follows the law of conservation of energy.
Without eating or drinking, how can something come from nothing?
But this is not the principle behind the restoration of qi.