Many designers don't actually play the game as much as you'd think, it's about the ability to look at what works and figure out why, I could play CS:GO for 60 hours and make a good level, you just look at examples and figure it out, A part of what makes showdown interesting, is you will be discovering what makes the mode great, developing games is hard, it's even harder when they are not complete yet. This is very similar to working in a development team, not on games that have been proven long ago.
The point of any contest really is to better yourself, we will always be considering good levels for the market place and the game, even after the contest has finished.