The divide here seems to be between those people who think PND (or perhaps any depression) is caused primarily or only by changes in hormones, and those who think it's more complicated than that.
If it was caused only by hormones at birth (presumably both c-section and vaginal birth), then sure -- it would only affect women who had just given birth.
This seems too simplistic though. If PND was only caused by hormone changes, then surely it would be easily 'fixed' by just giving a hormone injection or something? It's not that easily fixed -- so it must be the result of much more complicated mix of things: probably environmental and biological and social causes all bundled up together.
Plus if there is only one biological cause, then why do some women get PND at one pregnancy but are fine for another?
I say this as another person whose male partner had very serious, acute, crippling PND for three months, after a birth that was traumatic for both of us. It was very, very different from other depressive episodes he has had before or after, and exactly fit all the symptoms of PND. Until we were able to realize what was happening with him and get him helpbecause neither of us had ever heard of male PND beforeit was a frightening time for both of us.
He was (and still is) very hands-on with the baby from day one in fact, he's a sahd, so has always been the primary carer. The idea that he and other men who get PND are just being 'moody' or slackers because they don't really want to look after their children is insulting and damaging, because it perpetuates very harmful myths that stop families getting the help they need.
A final point: afterwards, we talked openly about our experience with other couples we knew. Turned out a lesbian couple we knew who had recently had a baby had had the same experience: the woman who hadn't given birth had PND, which went untreated for ages because it wasn't being looked for.