There is a difference between engine code and game’s code. There is definitely custom code written specifically for the game. And that code should be published too. And it should be licensed too. And at the moment the way it is published kind of makes it unclear whether the game is libre or not.
MIT is allow to be closed, so it’s free but not libre. I know its pedantic, but it’s a very important distinction.
MIT is still libre per GNU definition. https://www.gnu.org/gwm/libredocxml/x15.html I assume you mean copyleft for software license. closest thing to copyleft in creative commons is cc by-sa https://pitt.libguides.com/ld.php?content_id=18024633.
There is a difference between engine code and game’s code. There is definitely custom code written specifically for the game. And that code should be published too. And it should be licensed too. And at the moment the way it is published kind of makes it unclear whether the game is libre or not.