Mounting a FileVault-encrypted backup on another Mac

[jmsgwd]

Hi, I successfully made a final backup of an old iMac using SuperDuper. Since the old iMac was running macOS 10.15.7 Catalina, SuperDuper is unable to write to a pre-encrypted volume. So instead, I configured it to "erase, then copy", which generated an unencrypted bootable backup. I then booted from the backup and enabled FileVault, being careful to note down both the recovery key and the password of one of the macOS users.

Before I go ahead and securely erase the old iMac, I decided to verify that the backup data is accessible on my brand-new MacBook Pro M3. When I mounted the external backup volume, I was able to successfully decrypt/mount it using the macOS user's password - but not using the FileVault recovery key.

This is not an issue, because I know the macOS user password and I've confirmed that the data is accessible. However, I'm curious to understand why the recovery key didn't work. I thought the whole point of FileVault recovery keys is that you can use them if you forget your macOS user password. Any ideas why it didn't work? Thanks!