Earlier quoted context omitted.
He's a developer, not a telepath. In tests like this you make it clear what's allowed - "use the standard library, no external dependencies" etc. Because in a normal working situation, his solution is perfectly valid. If I were interviewing him it certainly wouldn't be a dealbreaker. I'd probably want to discuss more about pros and cons of in-house vs using libraries, because you tend to find out more about the skill…
In an interview you are trying to show that you understand the problem. Not just get the right answer.
One has to ask, but then the mere act of asking might also annoy some interviewers.