Not FOSS? That's a real pity.
I learned to program on 8 bit micros, and created a rough boot-to-BASIC experiment ~15 years ago following the JeOS pattern. So this project sounds really exciting, but I'd be more interested if I weren't restricted to being a passive end user.
I agree, it's a shame it's not FOSS.
Besides UEFI, it has a Windows GDI port, but if it were FOSS one could add SDL backend for example and then one could compile it for Linux and Webassembly as well.