Gmail does not literally unsend SMTP messages. To give you time to undo, Gmail delays sending the message for a few seconds. So if you don't select "Undo" within the time limit, your message will be sent.
Yes, that was a nice strong statement but you are wrong, and the other guy is right. When you send a message from Inbox, you can cancel it, for a few seconds. It doesn't bother actually sending it until the Undo button has disappeared.
Once it is sent, it is gone.
I always have fun with those "Please delete messages" from Outlook sent to my email client.