Piefed is a different piece of software from Lemmy, and an app supporting one will not necessarily support the other. They have different APIs.
Eternity is a Lemmy-specific app, and one that has been unmaintained for around a year, so I wouldn't expect Piefed support anytime soon. I recently begrudgingly switched to a different app despite eternity being my favorite user experience, because it doesn't support image proxying (which lemmy.zip uses) and will probably not be updated again.