I initially thought there was a word missing from the message as well. I think the setters (it's a collaboration) have been rather wily here. They knew that some solvers will remember the classic solution from encountering the problem previously, and that others (i.e. me!) will look it up online. Therefore they opted for a different solution which isn't as elegant as the classic one but is original and has to be worked out from scratch.
Didn't we have a Listener based on this in the last few years?