First, the security argument is nonsense in my opinion. This "feature" only prevents an attacker from flashing a modified, malicious BIOS on to the server.
But: If an attacker manages to flash a new BIOS to your server, you're already lost. That either requires physical access (which is bad), or access to the OOB / BMC / IPMI (which is equally bad, because those usually have a remote KVM feature, so you could e.g. boot the OS into recovery mode)
It does not prevent any other attacks, because you could still swap out the CPU. The servers usually just quietly burn the CPUs, so you wouldn't notice if the CPUs were replaced by an attacker.
Second, this produces a lot of unnecessary e-waste. About 99% of all hardware (except HDDS) from datacenters is sold on the second hand market. Locked CPUs are essentially worthlese, especially if buyers or sellers don't know and throw the CPU away because they think it's defective.
Third, this opens up a MASSIVE attack surface. Imagine if somebody finds a bug im the PSP (Platform Security Processor, a CPU inside the CPU that handles the locking thing amon g other things) and is able to burn arbitrary keys into the CPU. The attacker would randomly generate a key and burn them into the CPU. You could permanently kill an entire datacenter with that within seconds.
Or if somebody manages to write a malicious BIOS version and flash it to servers which usually don't have a locked BIOS. This BIOS version would also burn a random key into the CPU with the same result: You can easily permanently destroy an entire datacenter.
I think this is just AMD's greediness again in the cloak of "improving security"