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 skills and qualities of a developer from human conversation than scribbling on a whiteboard.