I found this very enlightening. PND isn't caused by formula, it's increased by using formula after wanting to bf. So it isn't biological it's environmental, and imo caused by the ridiculous pressure to bf and the notion that the only way to be a good mother is to bf.
I can personally identify with this. I ended up with mild PND bc I was so crushed I couldn't bf.
In other words, women who wanted to breastfeed and did = low risk of PPD. Women who wanted to breastfeed and couldn’t = high risk of PPD. The researchers stress “providing compassionate support for women who had intended to breastfeed but…found themselves unable to”.
This does not prove that breastfeeding cuts depression risk. It proves that women who had a goal and met it tend to have lower rates of depression. It does not prove that there is a biological reason that breastfeeding may be protective against depression. That may indeed be the case, but then the depression risk would have been similarly high in women who never intended to breastfeed.