It is not quite clear from either the article or the linked announcement.
Do they replace firmware blobs with open source alternatives or are they just breaking anything in the kernel that requires proprietary firmware blobs?
Taken from the linked announcement, it appears not:
The GNU Linux-libre project takes a minimal-changes approach to cleaning up Linux, making no effort to substitute components that need to be removed with functionally equivalent Free ones. Nevertheless, we encourage and support efforts towards doing so.
So basically it’s the user’s or maintainer group’s job to select replacement code/libraries/drivers and how they do that is up to them.
Gotta start somewhere.
It’s also good because if something bigly anti-free happens, people have a place to jump ship without starting from scratch.