The default setting is to only allow booting from your current macOS installation or from another macOS installation that is signed by apple and currently trust

The default setting is to only allow booting from your current macOS installation or from another macOS installation that is signed by apple and currently trusted (requires an internet connection to verify). That can be turned down to either accept any OS that was ever signed by Apple, or to boot whatever you want it to. The third option should let you attempt to boot Linux, but like you said, whether the driver support exists is another matter entirely.