I'm saying that ALL top 10-20% of CS grads will be able to answer questions at this difficulty level in an interview scenario (accounting for nerves and so on). I'm not limiting it to them (i.e. the "only" quantifier).
I know people in the bottom of CS grads (judging by GPA) that worked remote jobs or worked on personal projects during uni that are very talented and would be able to solve this in less than five minutes. But there are also people that merely "got by".
> I’d guess I was getting a random selection of candidates.
I stand by my opinion that if you were recruiting for a software eng role (and not some random front-end position), then blacklisting anyone who failed this problem would be reasonable.