Hi @mif4n. Thanks for the comment!
I will consider adding that as a feature if more people express interest :D
For now, you can programmatically set the difficulty in script.rpy as follows. A shorter movetime (in milliseconds) and a smaller depth will reduce the strength of the chess AI. Read more in my GitHub README and feel free to reach out with any more questions :)
$ movetime = 2000 $ depth = 10