The author seems to have missed the fact that iMessage only lets you send non-sms messages when the other contact is online (blue "send" button), otherwise it's sms only (green button).

They never said you'd be able to send free SMS from an internet connection, so that table is kind of obvious. In the end, the user doesn't have to care, it just works. The only case where you might lose a message is if the recipient disconnects right before you send it, and even then you'll know it wasn't delivered. FaceTime is in much worse waters.