Rogue, Kitty, Professor Xavier and everyone else are copyrighted characters of the X-Men franchise, a trademark of the Marvel Entertainment Corporation, and Oni doesnt own any of these. Therefore, he can't SELL his game under any circumstance or else he would have legal trouble with Marvel and Disney. He can drawn these characters tho, under the fair use laws, since it is a parody that doesn't try to pass as the actual thing.
In this situation, Patreon works as a kind of a loophole since Patreon's terms of service states that when you support someone you aren't buying a product, you are supporting his work. Oni's "work" is to develop this game.
TL;DR When you pledge money to Oni you aren't buying a game, you are supporting his work. He can't sell his game.