Secret moves in chess?

I once heard someone say you should never play a move in chess based on the assumption that your opponent doesn't know your plan.   iow, if you rely on the ignorance of your opponent, you weaken your own game.  If you assume that your opponent knows what you are going to play then it forces you to play the best possible moves.

Does anyone agree?   Is that even possible?  I know there are  players who could tell their opponent what they plan to do and still beat them -- like a great boxer who calls the round and punch beforehand.. or Babe Ruth pointing to the right field fence before the pitch.   But if it were possible between chess players of equal strength then wouldn't playing against one's self be more useful?  After all, the main problem with self-play seems to be that you already know your 'opponent's' plan..

Is there such a thing as a 'best move' that doesn't involve anticipation of an opponent's preparation or foreknowledge?