I know that the link you offered links to https://bombich.com/blog/2020/05/27/bug-in-macos-10.15.5-impacts-bootable-backups-weve-got-you-covered but I thought folks might want to jump directly to the source.
Thank you for raising awareness @ari!
I am no authority on the technical details reported by Bombich, the makers of Carbon Copy Cloner (CCC) but it's interesting that Apple makes the claim that a Mac can be started up from a Time Machine backup when some requirements are met and depending software and hardware configurations.
Source Quote, https://support.apple.com/en-us/HT201250
In OS X Lion v10.7.3 or later, you can start up from your Time Machine disk, if necessary. Press and hold Option as your Mac starts up. When you see the Startup Manager screen, choose “EFI Boot” as the startup disk.
It's reinforced in this article as well, https://support.apple.com/en-us/HT202796
The quote below mentions "you can also use this method", namely Use of the Startup Manager. (Pressing and hold the Option (Alt) key immediately after turning on or restarting your Mac triggers the Startup Manager.)
If your Mac is using OS X Lion 10.7.3 or later, you can also use this method to start up from your Time Machine backup disk. Startup Manager identifies your Time Machine backup as ”EFI Boot.”
The same second link has some gotchas at the page's end that I will repeat here for posterity: https://support.apple.com/en-us/HT202796
If you can't select your startup disk or start up from it
Check for these possibilities if you can't see your disk in Startup Disk preferences or Startup Manager, or your Mac won't start up from it.
Check for a compatible operating system on the startup disk
Make sure that your startup disk is using a version of macOS that is compatible with your Mac.
To start up from an external disk with macOS Catalina 10.15 or later, the disk must connect via USB-A, USB-C, or Thunderbolt, not FireWire.
Check startup security settings
If you're using a Mac that has the Apple T2 Security Chip, check the settings in Startup Security Utility. These settings determine whether your Mac can start up from another disk.
Check for Option ROM firmware
If you're in Startup Manager and can't see a third-party startup disk, the startup disk could be using Option ROM firmware. To enhance system security, Mac computers with up-to-date software don’t show devices that use Option ROM firmware until you load their firmware. To do that, press Option-Shift-Command-Period while in Startup Manager. If your startup disk appears, do that each time you want to start up from it or from another disk connected to it.
If you're using a firmware password, the ability to load Option ROM firmware is disabled as an additional security protection.
Whether any of these ramblings on my part add to the discussion, even tangentially is mere ambition.
This post was modified 4 months ago by Alex Santos