I'm not a medic, so I wouldn't know how much of PND is understood to be hormone-based, or why that is important, or how it's produced in the body, or whatever.
But I do think a distinction between the experience of someone who's given birth, and the experience of someone who hasn't, is fundamentally useful. So, no matter what PND is, I think we need a term that maintains a sense of that distinction.
Having a newborn is a huge shock to the system, and for most people it means sleep deprivation, anxiety, maybe financial stress, maybe all sorts of frightening unexpected possibilities about how your life is changing.
I totally believe anyone in close contact with a baby could develop real problems in that situation. I also think that being the partner of someone who's just had a baby has its own stresses, and they're real and important too. Just at the outside edge, but a not-trivial number of people who have babies come quite close to dying - if you'd seen your partner nearly die in any other situation, you'd be offered counselling and possibly some interesting sedatives for a day or two.
But it isn't the same thing as having physically given birth.
My partner had PND, and I could see it, and the midwife we eventually saw could see it, and she couldn't see it at all. She maintained she was absolutely fine for months. She really couldn't understand that she was actually frighteningly not herself - to the point of being paranoid about everything and basically not functioning. She thought she was fine. Or that it was 'just' sleep deprivation. My partner is one of those people who has massive issues over mental illness and she constantly wanted to tell me she was fine because 'you're sleep deprived too' or 'but you're scared too' (we had a very sick baby for a bit). I was, but I wasn't remotely in the same boat.
The only way I could get through to her was to keep reminding her she'd been through a huge physical and hormonal rollercoaster, and it was not strange that she was struggling with something very different from what I was struggling with. If you do away with the distinction between her situation and mine, you make it impossible for a woman who's like her to recognise that she is struggling.