This is all part of the "inclusion is always good, exclusion is always bad" bollocks.
We could, for example, decide that anything with a mood component is included in bipolar disorder. Depressed? You're bipolar. PND? You're bipolar. But unless you want people given inappropriate treatments, you'd then have to subdivide bipolar disorder, and make it so that people with, say, PMDD are excluded from some of the subdivisions of the new, inclusive bipolar disorder.
I want to see a good argument for why PND should be opened up to people who have not recently given birth, as opposed to the traditional inclusion criteria which are in the name of the disorder. Not just the "appeal to authority" fallacy you're pushing, but an actual argument for why it makes logical sense and why it does more good than harm to make "PND" inclusive of all these other circumstances.