I'd probably fail that from lack of experience, but it does seem to be a great approach for someone a bit more seasoned. That shows a lot more about someone's software engineering capabilities than short tricky problems.
Some one more seasoned would not work for free during a weekday.
See, I can play that game too.
Besides, this was a contrived problem. If you want to say "I refuse to solve any coding problems while interviewing for a software position", then fair enough. Good luck finding one. I think this was a total of perhaps 3 hours of my time, and was good enough to get an onsite? And it was time I -enjoyed-. Unlike most phone screens, timed interviews, etc.
I think the grandparent is reacting to "Untimed. They gave me a weekend" which made it sound as if the interview took all weekend which a more seasoned programmer would be less likely to do.