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Correct, and it's Linux-only and more hardcore FOSS (i .e. they don't have any blessed way to use non-free software).

I'm not sure why it's being sold as an alternative to Nix/NixOS

If you regard it as an alternative to NixOS rather than Nix the package manager then both are Linux.

Some people want FOSS only, some people dislike systemd, some people like Scheme....

Because it's an alternative which guarantees freedom?

just not freedom to use propriety software

It's easy to use proprietary software with Guix and nearly all users do this.

Cool, I didn't realize I could use it on my Windows or Mac machines. That's actually what's been holding me back.

Only Free Software guarantees that you have the final say in a matter of using arbitrary proprietary software.