You don't use target display mode on the more recent ones with thunderbolt (usb type c) input -- continue reading that confusingly-written page.

However in either case if your iMac is kaput you don't have access to the display -- the iMac has to be able to at least start their boot roms, though not boot macos, to make this work. Then again, modern external displays also have to be able to boot their firmware so perhaps I should't consider this a limitation.

Are you talking about using Target disk mode to boot your imac off the drive in a mac mini or MBP? because that will still use your imac's processor & ram, etc.