I don’t think that Libby itself is. There’s DRM and while there is probably a way to strip it, I don’t think that is easy and/or publicly shared. But Overdrive, which is Libby’s predecessor, allows for DRM free MP3 downloads. But they’ve been trying to sunset Overdrive for a long time. The Windows desktop program needed to download the MP3 files is no longer linked on their site, for example (but is still downloadable if you know the exact link). I’m honestly not sure why it even still works unless it’s to comply with some ancient contract they have with a library somewhere.