As in yeah there's precompiled binaries in this. But it's audited and each binary itself has a link to build instructions. What they are not doing is actually building everything from scratch in their build process. Ok that's a pain to do and i get it. But... i don't see anyone slipping in an unaccounted for binary here right? If every binary itself has a "here's how to build this from scratch" documentation and source it seems ok to me.
As in it's completely source buildable with no unknown binaries. They just don't have a single 'build' that pulls all of these in and builds them at once. Instead you're following the build instructions for each part, creating libraries that you then link together at the end. This is due to the pain in the ass of cross-compiling Linux/Windows/UEFI binaries all in the one project. It's pretty reasonable.