Quoting your comment: "You definitely don't want to hire the person who thinks - bizarrely - that using library functions is a sign of weakness."

So true...I've failed interviews, because the interviewee did see using library functions as a sign of weakness.

I just wish that in those cases the interviewee gives feedback and allows you to rewrite instead of just failing you. I mean in practice nobody writes library functions themselves unless absolutely necessary, but I get that for some positions you have to demonstrate that you can write lower-level code if you have to.

I think that it was probably a poorly designed question, but surely you could throw the interviewer a bone by giving a custom answer after they reject the library.