I think what GP is saying is that this test correlates with things like 'fitness' and 'balance', which themselves are causally related to health outcomes. But training to balance on one leg doesn't mean that your 'balance' or 'fitness' overall improve.

It might be that one legged balance doesn't improve 'balance' and so doesn't improve your health outcomes. Or 'balance' is irrelevant, and it's just that 'fitness' means you're less winded at the top of the steps, and so you're less likely to fall down at the top.