The point is that deadlines are bad for the people doing the work when they can't influence the amount of work. Not that deadlines should be removed completely, but they should be moved to a level where the amount of work can be influenced, namely project/product management. They should then, completely independently from the development team, adjust the backlog and/or deadlines in case the project schedule starts to slip. If the deadlines are needed because there are doubts that the dev team is not working as hard as they reasonably can then there are bigger issues that any project management method cannot fix.