That, of course, applies only to SMS/texts and not to IM/Instant Messaging. In the latter case there is no practical distinction between a message being received and opened - the clue is in the name, Instant.
I think the distinction would be that on the senders phone, it will say either
delivered or
read. A message could be delivered and seen/read on the home screen of the receiving phone without opening the app thereby sending off the pong that tells the sender it has been read.
The instant part of instant messaging means that imessages are going over the internet (3G, 4G, Wireless) and not the normal phone networks. Normal networks can delay messages depending on the networks capacity/workload. This can be demonstrated for example at busy periods such as New Years Eve etc. The internet has a vastly greater bandwidth than the normal mobile phone network, so messges pass more freely, or to re-phrase that more instantly.