There is no hotseat for a single controller, that’s correct. You can play both sides with the keyboard, though.
Unit Playground’s purpose is to immediately get into a map with all of the characters to test their movesets. It was originally a debug map for testing various interactions between units and I left it in for people to experiment with. Adding a drafting step for that map would slow down my ability to test units out when iterating quickly during development, so I left it out for this one.
Thanks for playing the game!