"The game must be run in windowed mode via dxwnd or similar software"

What is the purpose of this rule?

This game does not run well with modern hardware when run as is, most notably with the higher FPS that effects the physics of the game and the speed of the in game timer. That rule is in place to help normalize the performance of the game between runners. The expectation is that you use it to cap FPS at 60.

I'll add a note to the rule to make it a little more clear.

That makes sense. I have a pile of old hardware and can easily run the game with it, but this is a good way to keep performance consistent between runners.