I hope the author isn't making an argument that network partitions are somehow the only consideration of whether a message is delivered. Even on a single user single process machine, delivery can't be totally guaranteed.'exactly once' delivery sounds like 'guaranteed perfect one time delivery under all circumstances', which is a dumb way to rephrase an idea whose literal translation is to deliver something one time and not more or less.

In other words, of course you can deliver something exactly one time. Just not every time.