I think you're correct--someone else in the comments pasted the FTC's definition of "transactional," and the transaction in question may be an already initiated commercial transaction, not an interactive transaction between two parties on email like I had thought.
CAN-SPAM likely doesn't apply to the message in the first place, but even if it did it does not specifically require that you can unsubscribe by replying. That's just an example of one method they can provide.