You need to read better. It says it reduces occurrence of myopia in a population not that it cures myopia when an individual gets it.

Sure if you have very mild short term myopia caused by eye straining you can get vision back by training your eye. But with kids it’s about how the eye develops when it’s still growing. When kids eyes grow too fast they grow less spherical and that is what causes myopia and that is the kind that you can never cure. Going outside means kids are getting more sunlight in their eye which will slow down the growth and thus their eyes will grow more spherical which means they don’t develop myopia. Playing outside won’t cure myopia but it will reduce the chance of developing it in children.