message 1 -> attack!
message 2 -> sure!
message 3 -> I got your sure! At this point, general A knows the message got through
message 4 -> I got your confirmation! At this point, general B knows that general A got the reply
message 5 -> I got your confirmation confirmation! At this point, general A is sure that both parties know when the attack is. General A knows that the confirmation got through, and Gen A is good to go.
message 6 -> conf^n! At this point, General B knows that the confirmation got through, and Gen B is good to go.
message 7 => icing
Even if message 6 doesn't get through, message 2 and 4 have, so general A knows that both generals know. Message 3 tells general B that both generals know - all that's missing is the nice-to-have confirmation.
You don't need to keep on confirming the last message ad infinitum, because that doesn't add new data about the original message and who knows it. If messenger 77 were to be captured by the city, it's not like the generals would call off the attack because they weren't sure of the timing info.
Perhaps another way of putting it: this isn't a continuous string of information that gets added to. It's a set of discrete parcels of information. The additional messages only add information about the veracity of the later parcels, not the initial ones.
A and B must not attack alone, i.e. A must not attack if B is not going to attack, and B must not attack if A is not going to attack.
Now, message 1 (from A to B) says "attack!". So B knows that A wants B to attack.
...but A mustn't attack alone, so A won't attack unless A receives B's confirmation. So B sends message 2 ("sure!").
...but B mustn't attack unless B knows that A is going to attack, and A will only attack if A has received message 2. So A has to send another message to confirm receipt of message 2.
...and so on.
None of these confirmations are simply "nice-to-have" - if you need to be 100% sure that you're both going to attack, you need to ensure that every confirmation you send has been received by your peer.
You have that confirmation of agreement in the third round of messages. Receipt of the fourth message proves that both sides have knowledge and have agreed, aware of the other's intent. That's the message you have to prove has arrived for surety. Once you've done enough messages to prove message #4, you're good to go.