Good point. What I mean to say is that the question of "who's smarter, rich or poor?" is not relevant to the study in question, and can't explain the results.
You are suggesting that maybe poor people just have lower IQ, but this study doesn't support that.
It shows that cognitive performance between rich and poor is the same when they're asked questions which both groups consider easy and/or non-stressful.
The study is not trying to prove that worried people do worse on tests. That's well-established, not news to anyone.
It's demonstrating that day-to-day financial decisions like "should spend $1500 to repair my car?" produce enough worry in poor people to have a strong cognitive effect, wheras they don't have that effect on rich people.