By contrast, if I suddenly decide that I want more money, I have a number of avenues by which I could arrange that, at least on a small scale. It would be mistaken of me to conclude from this abundance of available financial opportunity that everyone chooses to have the amount of money they have, and that people with less money are choosing to take fewer of the equally abundant opportunities they share with the rich.

This all seems true with the exception of 'by contrast'. You seem to have clearly illustrated a similarity between weight loss and financial gain. There are things that are under people's control but which things are under a given person's control vary by the individual and the circumstances. In both cases people drastically overestimate the extent to which the outcome is a matter of 'choice'.