Also, any test that you ace has not completely characterized your actual skill level; all it tells you is that you're above the skill range measured by the test. You might be a little bit above, or far above, that upper bound. Maybe they've decided to have one really hard question that it's unlikely anyone will answer correctly, but if they get someone who can answer it then they know they've found someone really suited for the job.
Or it could be a mistake, and it was just bad luck that your friend got an interviewer who doesn't know how to interview, and asked a question that about a subject that they're intimately familiar with, but is too hard compared to what is asked by other interviewers.
One may argue that a Phd student working in genomics must know the Monotone Queue variant because it MAY come up as a research implementation in his/her work. But this doesn't hold true for general software engineering folks.
I am a Phd student in Computer Graphics and so it is a reasonable expectation that I should know more about linear algebra than my peers. But if you extrapolate it to ask extremely hard research level problem it kills the purpose.
Or it could be a mistake, and it was just bad luck that your friend got an interviewer who doesn't know how to interview, and asked a question that about a subject that they're intimately familiar with, but is too hard compared to what is asked by other interviewers.