UpToDate is paywalled but here is a relevant excerpt for you on postpartum paternal depression which is a very real diagnosis:
New fathers may develop depressive symptoms or disorders. Postpartum paternal depression can interfere with paternal-infant bonding and is associated with adverse effects upon child development.
Prevalence — Based upon studies that used diagnostic interviews, the estimated prevalence of postpartum paternal depression ranges from 3 to 5 percent:
●One study interviewed fathers (n >2000) of infants and found that the prevalence of postpartum major depression was about 5 percent.
●A study of a clinical database that included new fathers (n >86,000) found that by the time their children were one year old, an episode of postpartum depression had occurred in 3 percent.
Given that you acknowledge depression I expect your misunderstanding is the fact that postpartum depression is defined as unipolar major depression occurring within 12 months after birth, it is not sex specific and can be either maternal or paternal.
Just some unsolicited advice for you: your comments read as antagonistic and arrogant, and are neither going to get you useful information nor make you any friends. If you're genuinely curious about how this is possible, why not ask good-faith questions about their experience, and then make a judgment quietly to yourself? Instead, you're invalidating them from a position of ignorance. Even in other cases where you might be right, you still look childish and insufferable.
Although I agree that a different name for this type of depression could be useful in a medical context (as the root cause of post-partum depression is likely to be different in the father vs the mother, so the distinction does matter), "post-partum" as a modifier can clearly apply regardless of who is going through the "partum". e.g, the father is experiencing depression after the birth of his child, therefore "post-partum depression".
Edit: and as others have pointed out, a separate name is probably not needed as medically this is generally referred to as "post-partum paternal depression".
