I'm not sure that really works out in practice because there are all sorts of ways to get an OS up and running in an "insecure" manner, running in a VM or other contrived environment for example. This isn't about protecting against compromise in any arbitrary case, it's about a very specific case of booting the primary OS on bare metal. You could certainly "secure boot" a small core system, windows or linux or whatever, and use that to make a contrived environment for booting your arbitrarily compromised OS (linux or windows or whatever) and I think this case is out side the scope and threat model that Secure Boot is intended to address.
You can't win them all 8-)
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