Thanks for the question, I just added this answer to the FAQ: "You are allowed to create a game for any platform; however, it is highly recommended that you create a game for the browser and/or as a Windows executable (Mac and Linux versions are a bonus - the more platforms, the better). Games created only for other platforms are less likely to be played by our YouTubers/streamers and other participants, and will receive less feedback and ratings as a result."
So you're free to make an Atari 2600 game, but I don't think many will be able to play it; if you can make a version playable on Windows too that'd be better!
