They are not orthogonal. If you can install Linux, then you can sideload apps on macOS. It might involve invasive XNU patching with a hex editor, but it can be done.
It's not that involved, by the way. There's a command that you run to turn off gatekeeper [1].
I think the OP is pointing out that installing Mac apps from places other than the App Store is not what most (if any) Mac users would call "sideloading". We just call it "installing." :)
Exactly, and this goes beyond mere self-signed code (Gatekeeper) and into things like SIP and private entitlements. As long as the user controls the bootloader, the user can also patch the OS to do whatever they want.
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.