Grub can load booster images, the issue is about incorrect grub.cfg generation.

What they're saying in the issue is that grub-mkconfig will not create a correct "Arch Linux" menu entry for booster, but if you go to "Advanced options" and choose the "booster" menu entry it works. I can confirm this. It happened on the system I'm currently using.

Specifically, the problem is that grub-mkconfig does not add the booster image to the initrd of the default menu entry. You can add it manually. For example I had to change this

initrd  /intel-ucode.img

to this

initrd  /intel-ucode.img /booster-linux-zen.img

If I recall correctly this issue was not present last time I set up a system with booster. It might be a regression or maybe it only happens in specific system configurations.