probably true. Maybe an true open-source eReader should exist.

They do exist, since a pretty long time. I bought a pocketbook beginning 2010s. https://pocketbook.ch.

If you mean epub reader software Calibre and a bunch of others exist since pretty much the beginning of epub

Arguably

https://play.google.com/store/apps/details?id=biz.bookdesign...

should ~~be~~ EDIT have been ENDEDIT opensource --- it does at least work to support Project Librivox (or at least that's my understanding)

Seems to no longer be available (see below)

I'm getting "We're sorry, the requested URL was not found on this server." if I go to the link

I believe the app is discontinued and the reason I can see the page is that I am on record as having downloaded it.