It is compatible. You need to jump through the email confirmation hoop before it’ll actually work with the app though. So if your account is new, that’s probably the reason.
Besides that, it could just be the mass influx of new users that are fleeing Instagram.
Because news broke that Meta lobbied for the TikTok ban. So people are now boycotting Meta platforms like Facebook and Instagram. Since Pixelfed is an Instagram clone and recently launched the app, it has gained a ton of popularity. The timing of the app launch honestly couldn’t have been better, because it gave all of the fleeing Instagram users a nice convenient place to land.
I was actually searching for this exact idea like two weeks ago, and came up empty handed. Sometimes things do work out, I guess.
For the unaware, there’s an app called Prologue, which allows you to listen to audiobooks on a Plex server. Plex notably doesn’t have native audiobook support… The first-party PlexAmp app doesn’t support chapters, for instance. You can have the library remember your listening progress, but that doesn’t help much with a book audio file is 12 hours long. As a workaround, Prologue uses Plex’s service to access the files, then is able to read the chapter data from the m4a/m4b file.