"iCloud lock" is synonymous with "activation lock", most of the time.
Your second point is where it gets a bit ambiguous. What really matters is whether "Find My (Mac)" is turned on for the device.
If it
is, you can't do much unless you know what to replace in a T2 SPI flash dump, which nobody here seems to want to share

If it's
not, then you can simply put the device into DFU mode and use Apple Configurator 2 to wipe everything, then use Internet Recovery to reinstall macOS.
But it's very likely Find My is enabled for the MacBook in question; I think that's the default for any Mac (unless you disable location services during setup? which you'd have to do explicitly)
My question:
What do you have to replace in the T2 flash to "unlock" a Mac tied to an Apple account? Just the serial number? Or the MLB + UUID + ROM as well?
Or is that not possible anymore? I don't see how that could be the case; as far as Apple can tell, it's an entirely different Mac, right?