Implementing a stdlib isn't a thing you do every day. Presumably GP was referring to more common situations, where using the stdlib is (usually) preferable to rolling your own solution.
They were interviewing for Google. It's not uncommon to have to implement a standard library function (or not be able to use one) in many contexts at Google. Besides, the question is just the most basic setup to assess how a candidate would fare in comparable situations.
Is it though? Because the answer to "suddenly I need to implement a standard library" is "I google the accepted solutions" and if I don't have Google then I goto the library and check out a bunch of old CS books.
And then the interviewer's follow-up question should be "fine, since you don't want to indulge me and implement a solution to this toy problem, I'll ask you to implement a much more complicated function which doesn't exist in any library!"
