I don’t see any reason why you’re wrong in theory. But there are plenty of institutional investors who can’t go to cash and have to stay invested as their clients expect that, so the next best option would be to go to short duration. There are plenty of instances in the text where you can hedge your long duration by going to synthetic cash instead.
As usual S200magician is right. Coming from an asset manaement background “cash” is t-bills or good shor term paper. It all has duration, even floaters do with rates where they are. Just forget the idea of cash as the dollars in your wallet. Money in a fund is always invested in someting.
Also from a curriculum standpoint, moving to cash completely could be a strategic asset allocation shift, while tactical is probably more appropriate in tilting the portfolio.