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.

AFAIK Eternity is abandoned. Hasn't had an update in over a year.

Blorp supports both lemmy and piefed. Thunder seemed to drain quite a lot of battery. Blorp is pretty basic but fine for me.