Why do you think this would have adverse selection against the top of the pool? I can clearly see how it would act a barrier to application for the bottom candidates (a good thing for the company), and I can see how it would reduce the number of applicants who are applying "everywhere they can think of" (also probably good for the company in terms of getting a more qualified [in the sales-conversion-rate sense] base of applicants).
In my experience, the very top of the applicant pools aren't applying to a particularly large number of places.
Although the top tier candidates may not be applying at many places, they are receiving many more first, second, or even third interviews. Thus, their timing is tight in a two week window near the end of school when they start receiving offers. Near-Bottom-tier candidates may receive less interviews and are incentivized to perform more desperate acts in order to receive interviews, and thus have more time for puzzle-based applications.
But as I said, it's not too big of a deal, if they would only post ahead of time how long puzzles would be active, so one could complete them before this tightly-timed period.
In my experience, the very top of the applicant pools aren't applying to a particularly large number of places.