As of yet, nobody has used reduce(). If they did, and especially if they followed it with a coherent explanation of what reduce does & how it worked (and convinced me they've not memorized reduce for a singular purpose, but understand it well enough to apply it generally), I'd definitely accept it.

If they can also write, or at least quickly approximate, the for-loop solution too, then I'd say I've gotten an excellent signal: not only can the candidate write a for loop, but he or she might also have some experience with a functional paradigm from somewhere.