It's not a foolproof way. Many thin women with pcos still have debilitating symptoms. I have a friend who has to lay in bed on pain meds once every few months due to cysts, and she weighs like 110lbs, tops. (TBF she can't take birth control due to it giving her migraines, but when she was on it, her symptoms were non-existent, so weight loss + hormonal therapy is a lot more effective than just one or the other). Being overweight does exacerbate your symptoms, however. So everyone who has problems with pcos at a thin weight would be much worse off if they were overweight.