Regardless of whether it's true, I don't see how this is relevant. The parent wasn't saying it's better to be poor, they're saying it's not helpful to not move. If you had a high paying, physically active job that you could do without injury, it would be better for you than any of the other combinations of pay ranking and level of activity. Likewise if the cultural ideal at the top was not decreased activity. If everyone had sedentary jobs and the cheapest food was the worst for you, you'd probably expect those with a surplus of money to be able to compensate better. Those without the need to compensate will be better off, but rich sedentary people will still need to.

They are normal weight but often very unfit.

They are on average more fit than poorer people.