What exactly is your argument? Why would a computer only be able to run Windows?

Secure Boot doesn’t restrict anyone to only windows. Even if we play with the idea that it’s impossible to disable it. You can still install some Linux distributions.

Anything can support secure boot, the question is, are the keys included in the BIOS so it can run that particular OS without loading extra keys?

I don’t even understand what you’re trying to say… You don’t need keys in BIOS to install either Windows, or Linux. The only purpose for the BIOS key is for users to be able to just boot up their new computer that they bought factory new WITH their OS of choice without having to go through extra steps of verifying your OS license.

But you don’t NEED a key in BIOS. You can still buy a key separately to set up Windows. Same goes with paid versions of Linux distributions, such as Red Hat.