it sounds like they consider a male seahorse a male because he produces sperm rather than eggs.
The female seahorse drops her eggs off into a male brood pouch, a little pocket the male seahorse has on the front of him that has a placenta in it, and then he fertilizes those eggs and carries the fetuses for a few weeks and then little seahorses flutter out when he gives birth.
there’s a video. and it’s a LOT of baby flutterhorses
https://animals.howstuffworks.com/fish/male-seahorses-give-birth.htm
Incidentally, “reverse kangaroo” is also a sex act that is prohibited by law in all of the states and territories of Australia, except Tasmania.