Adding the ticket number and it's title to the subject line of an email (unrelated to any reply thread from a ticketing notification) and sending it to the ticket creation inbox will not cause the contents of that email to post to the ticket listed in the subject line. The system is not able to just grab a number from the subject line and know that is the intended ticket to be added on to. They must be responding to a notification that has a reply token at the bottom so it can append properly to the ticket.
I hope this helps answer their question. Let me know if you or they have anything else we can assist with.