It is. MIT license on the code, CC-BY on the binaries and resources.
MIT is allow to be closed, so it’s free but not libre.
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.
Something can be Libre without it’s initial distribution being free (as in beer).
That’s how many distributions used to do it in the days when CDs and floppies etc weren’t exactly free.
The difference here is that the game is gratis. Its the source code that initially didn’t come with the game you get.
