I think the reason that I put that phrase on that event is that (perhaps to a playtester or just to me, I don't remember) it looked like it might be a Response Card, and I wanted to be clear that it is not. That may have been because players wanted to use it as a Siege of a Fort reaches Siege 1: you can't, because the Fort will fall before its your turn to play a card. So you have use the event when your Fortress (requiring Siege 2) is under Siege and gets to Siege 1.
... I always must use my whole action phase to play an event.
Not if it's a Response Card. So text is just reminding you that "Bastions Repaired" is not a Response Card.