Some of the Linux distros have signed bootloaders/kernels for SecureBoot on PCs, so it would be good if that same model could be allowed on macs.

Given the state of signed firmware, I think Apple is in the right - https://arstechnica.com/information-technology/2020/07/new-f...

I think that the two are at best tangentially related.

It removes some specific types of security, but System Integrity Protection is a "runtime" protection on OSX. It disables access to various files/etc even as root unless in the right context.

So it seems unfortunate to have to lower the level of runtime security for OSX, in order to enable booting unsigned software. As they shouldn't be strictly related.

My understanding is that if boot security cannot be established, it's impossible to assert runtime security either. Otherwise, a compromised code in the boot process can also alter how SIP behaves, essentially invalidating its security function.