I am not familiar with Macs so I might have been wrong from the very start, but if the owner put the key in the secure enclave then the same user should have no problem at putting the same key in the secure enclave of another Mac. If the user can't write into the secure enclave, then the Mac is protecting the data against the very same user that is supposed to own those data. It is protecting against wrench attacks too but what's the most frequent scenario?