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.

I like not calling carbs or even just highly-processed carbs "bad." Carbohydrates are just molecules. What's important is to know how the body reacts to these molecules--this can inform you on how much carbohydrate, by type, your body can tolerate.

The advice about fiber is totally bogus. Humans don't need fiber in their diet. The shit that fiber helps pass through your system is largely fiber... You might find this article interesting:

https://medium.com/mind-body-soul/fiber-what-is-it-good-for-99e5468ba7ea