My guess is that the key to decrypt the disk is stored on the disk, encrypted by a Microsoft-known key. This seems to unlock that copy of the key rather than the copy encrypted by your own key.

Though he did say to put the disk back in the original system in part of the instructions, so it might be TPM based. The way to check would be to try this on a system with a disk from another system, or with a wiped TPM.

TPM is not security, it’s security theatre. If you don’t need to type a password in or insert a device with a key on it during boot, then it’s not secure, period.