>>106156722every linux distro uses the same key (the 3rd party thing you can pay for, not the leaked key)
obviously the fact that you can just pay to get your binary signed means that if you trust that shim you also trust everything else using it
new pcs don't trust the 3rd party key by default, which the pdf i linked alleges and i think is what you're referring to
however, you can still make use of secure boot if you'd like to protect a computer from evil maid attacks, this would require generating your own keys (and not trusting the 3rd party one for the sake of completeness), see https://wiki.archlinux.org/title/Unified_kernel_image