I used to have a 2011 macbook pro. I couldn't boot USB media at all. It's not a new problem. After my optical drive died, I had to resort to creating my own installation media on a hard disk partition..

Guix rejects non-free software in their repos, so I would be unsurprised to hear that no contributors use Apple hardware.

I agree that what you are trying to accomplish should be doable and even non-trivial. This is definitely a disadvantage of having a more automated installation procedure.