It's 2 separate points, Apple explicitly said they allow sideloading (I wouldn't even call it side loading, it's just "install from outside the app store" and "

It's 2 separate points, Apple explicitly said they allow sideloading (I wouldn't even call it side loading, it's just "install from outside the app store" and "allow unsigned applications if you want") and booting any other OS is allowed, there is even some amount of Apple documentation on how to do it, and success with both BSD and linux. Drivers are of course an issue.