Being dissatisfied with something and being satisfied with something may be on opposite sides of a spectrum, but the questions you would ask if you're focusing on one end of the spectrum versus the entire spectrum are different. If you were to ask, how satisfied are you with your body? That would span the whole scale, but if instead you were asking questions like "agree or disagree to the following statement" and that statement is "I often wish I was thinner" then, disagreeing with that statement would not necessarily indicate that you were satisfied with the way you looked, just that you were not actively dissatisfied. I don't know the particulars of the study, but often times what is considered "confusing" is more aptly attributed to attempting to accurately convey concepts using specificity that may not be required in normal speech.

it basically is

the women with high body dissatisfaction are the ones who focus on low weight stimuli

"Not" dissatisfaction and satisfaction are not the same. That's too binary.

Dissatisfaction is a different headspace compared to satisfaction. If the study was built around dissatisfaction you can’t just reverse it and say it’s true. Double negatives are necessary sometimes to actually be clearer.