Basically processed food is much more dense in calories so you can easily eat more calories than you need. You'd have to eat a lot of vegetables and salad to get calories. The study is new, but the conclusion is ' hacker diet' - people eating processed food ate more calories than they burned so they gained weight.

If you want to lose weight just burn more calories than you eat.

Usually the easiest solution is a mix of better diet and some moderate exercise regime that burns some calories and increases your metabolism.