The installer does not load X or mesa.
The installed system with a login manager like GNOME may try to load a
driver on startup (like radeon or amdgpu) which in turn requires the
loading of a firmware which is missing from the linux-libre kernel.

If you want to confirm this, try booting another distribution that uses
linux-libre, like Parabola or Triskell.

-- 
Pierre Neidhardt
https://ambrevar.xyz/