It does not equate anything. It describes the correlation and steps to not waste huge talents. In fact it does say they do not know exactly why some highly intelligent people do not achieve much.

It is surprisingly easy to waste talent. For example, get a gifted person through standard school and they will likely never extend enough to achieve full potential, often by growing lazy and coasting. Or push them into a wrong field, they will do good but not exceptional.

See, if your niece was challenged enough, she would probably place more value on achievement and thus achieve more. It is a positive feedback loop. The correct approach is to give them as much challenge as they can take. Evaluating where this point is takes some major interpersonal skills and experience.

Emotional support is a necessary ingredient as well so that failures do not break them and they remain human.