If your reread the sentence you may note that I was careful to make that adjective redundant - sufficiently redundant as to border on absurd. "A rational agent will X or be irrational" is just silly. "A rational agent will X" would have been true but misses the point when talking about humans. That's why I chose to write "An agent will X or be irrational".

Indeed, I obviously didn't register the sentence properly, edited.