For what it matters, I am also of the opinion that the cheat should be public. Whether it spoils the game or not, it should be up to the player to chose what he wants to do with it. Nifflas' opinion on the issue would surely put an end to this discussion, but considering how open he generally is about his games (letting players have access to the save files and edit even the levels made by him), I can't think of a good reason he would disapprove.
However, even if Nifflas decides it is to be kept a secret, I do not understand why it should only be hidden in THAT particular level. It is not by Nifflas himself nor even an "orthodox" level, but one which heavily relies on brute force and, although I personally enjoyed it, is not for every player. This "The Explore Challenge" cheat monopoly status is, I think, rather unfair and unjustified. While it might have made some sense back in 2007, there is currently no reason to force this level upon people willing to know the cheat, with all the levels that are available.