No--you misinterpreted the results reported in the paper. The flexible diet didn't show a strong relationship with BMI--as in no correlation. In other words, those engaged in rigid dieting tended to have higher BMI, while those engaged in flexible dieting had a range of BMI scores (some high, some medium, some low.
Correct — correlation does not mean causation. However, any RD worth their soul would tell you based on their personal practice that the more restrictive an individual is with his food choices the more likely he is to overeat or binge. It’s one of those things that we don’t need more studies on. Whether observational or randomized controlled trial.