From my understanding of things, which could be wrong, fsflover isn’t mixing osi and fsf when they make a statement and link to the fsf for supporting statements.
Also, I don’t know the specifics of the drm audible is using, but I know that there are open source drm solutions. Free software being about freedom not necessarily free beer to me means that pureos and drm are not mutually exclusive as concepts. The specific DRM for audible may or may not be, but that’s different than implying the concept of DRM is.