Quote from the post:

"There's just a small problem.

It's not with the box analogy: as another commenter noted, it's a well known example example you'll find in great books like Simon Singh's The Code Book."

The problem is not the analogy, the "locked-box" analogy is fine. The problem is that the math is flawed. The math is not equivalent to the analogy.

I believe the question is how can you be secure if someone is listening to both sides of the conversation and the answer is you can't in this scenario.