Bizarre thing is above it says "Is encrypted: no" while it definitely is...
Found the command to mount an encrypted volume on https://apple.stackexchange.com/questions/42257/how-can-i-mount-an-encrypted-disk-from-the-command-line - but that fails as well:
% diskutil apfs unlockVolume 6EFC662A-8977-4341-A725-06B5C4E039D2
Passphrase:
Unlocking any cryptographic user on APFS Volume disk5s1
Error unlocking APFS Volume: Unable to get VEK info for an APFS Volume which is marked as encrypted in the IOKit Registry (-69461)
sudo fsck_apfs -n /dev/disk5s1
Password:
** Checking the container superblock.
Checking the checkpoint with transaction ID 6636.
** Checking the space manager.
** Checking the space manager free queue trees.
** Checking the object map.
** Checking the encryption key structures.
error: failed to enable crypto I/O mode for container /dev/rdisk5: Invalid argument