Yes, you are correct. That is weird.
What can cause this?
1) Perhaps re-using an old serial-number. Part of the system details are encoded in it.
2) More than one Clover EFI tree. During boot the verbose scrolling text is stepping through the boot process. It hits a MacPro3,1 system-definition and says "Whoa! You can't install Big Sur on this machine." That system-definition comes from somewhere. There's no AI involved here.
Check the root of your system drive to make sure there is no EFI folder there. EFI Backup is okay, but EFI on its own indicates a Legacy install at some point.
Check any other drives you may have attached to your system. If they are GUID drives they will have hidden EFI partitions too. Did anything get installed in there in error?
At the end of the day there's a reason you see what you're seeing.