Threat model is just trying to lower the chances of infecting the main drive even if stuff like games or software are from a “trusted source”.
Aside from getting an enitely separate system dedicated to just running pirated games which is expensive to do.
Unmounted drives in case of dual booting still leading to infections is what made me wonder about installing an OS entirely on the external SSD and physically unplugging other drives. Of course, as you said bios is still a risk. But, more just trying to lessen chances from trusted game sources by not installing right away from release to see if anything happens to other people the first couple of weeks. And just wishing to not intermingle the two environments.