Or one should spend a little more time at first to fully understand the problem, in both the mentioned case and the OP, use realistic data.

Writing code is very very often a big part of understanding any new problem. It's just that you should be quite comfortable discarding that code. Refactor mercilessly and all that.

Writing code that solves the problem is a huge way of proving you understand the problem.

Sometimes you can't have realistic data without writing some code first, that will allow you to gather that data.