The Guix (almost) full-source bootstrap

I'm happy that you quoted a relevant section of the GPLv2, but I don't think it invalidates the point I was making (perhaps I wasn't clear). Program 1 is GPL, requires Program 2 to build it. Even if Program 2 is "normally distributed", if Program 2 is also GPL then it gets distributed and the GPL applies to it independently. Hence a chain. But, source distribution is not just a GPL requirement, it is a principle. Thus Guix is, in a way, advancing a frontier of software freedom.