 He explicitly says that this is due to the understanding of the words above, not due to "la haraj" (principle that states no Islamic law can be the cause of extreme hardship) i.e.even if it does not cause difficulty, you are still required to fast a maximum of 17.5 hours

The consequence of the viewpoint of al-Shirazi (as well as Makarim Shirazi) is that if the fast is longer than 17.5 hours, you should revert to times with "mu'tadil" lengths of day e.g.Mecca or Karbala which could be 14–16 hours.