There's really no way to control for the lifestyle stuff or the "breastmilk from a bottle" vs. "breastmilk at the breast" vs. "non-breastmilk supplemented at the breast" etc. The only way they would be able to study this would be to enroll thousands of families in a longitudinal study, visit them regularly for like 30 years, fill out a super intense questionnaire each time (and even then, it would only capture what's happening with the family *right now* or *this week* - people don't remember their habits in sufficient detail usually to take longer histories), etc. It would be a major to-do.
I think the nutritional benefit is likely undeniable, but likely minor and may be obviated in some cases by people having terrible diets/taking medicine/having illnesses/being on drugs/etc. I was always more convinced by the social/psychological/logistical benefits: food is always right here, at the right temperature (benefit is taken away by having to work and pump though), feeding necessitates cuddling/soothing/eye contact, child continues deriving comfort and sustenance from mother's body which lengthens the high-touch nurturing phase, etc. I think it's sad that people aren't taught that some of the psychological benefits are accessible even if you're not feeding breastmilk. Supplementers are a PITA to use, but they can be really beneficial psychologically for mothers who wanted to breastfeed and aren't able to for whatever reason. I only used one for a little bit and had a dual goal of increasing milk production by increasing stimulation (didn't work), but it was really a sweet thing to do, even if in the end I decided it was way too much effort.