That's up to them. Clearly Anet felt it was too frustrating for a prominent feature of a holiday event.Really though, what kind of metric should they use to determine how easy or how difficult? Say if 100% of the playerbase gets it on the first try too easy, 100% of the players can never do it too hard.
We have a dev [puzzle designer] telling us us flat out it was a mistake to put that kind of puzzle as a major feature of an event. Can't get any clearer than that.