Thats what I thought as well, but encryption of sectors does not create “garbage” as far as the drive is concerned – a write to a sector by the file system, and a write by encryption are equally valid things at the drive level. That’s why an external “rationalization” is required.
As for spare space for garbage, no drive has enough to cover the case of full disk encryption. You’d need more space in the garbage block than the storage capacity of the unit.