I get what you’re saying, but this is what they call anecdotal evidence. Plus, the article doesn’t claim that a person’s choices can not affect their financial outcome, it says that [edit: “biased decision-making”] alone do not account for the amount of income inequality prominent in the countries studied.
What you’re saying may be true, but if a terrible fate befell one of you, you would both have the assets to weather it. However, someone in the lower income bracket would not, no matter what choices they made, for reasons many times beyond their control.