They still don't know why people get RA. One of the ways they try to study disease is with epidemiological studies where they try to understand risk factors, just like smoking, high blood pressure, diabetes, age, sex and family history are risk factors for coronary artery disease. RA is just one type of arthritis. This is an epidemiological study that says among other things, women will have a lower risk of getting RA the longer that they breastfeed. It doesn't say it will prevent RA, just lower the statistical risk.
I understand about hormones and pregnancy better than a man,even one who studies it,ever can.
Here's an anecdotal example for you. My grandmother didn't smoke,there was no family history of arthritis,and she breast fed two children,but got arthritis in her fingers ( which was a terrible blow,since she was a concert pianist)in her late 30s.
A very great deal of these breast feed your baby or.....junk science studies are junk science,with the outcome already decided upon,BEFORE the studies are even made.