This seems to clearly be a passion project. The author is being paid in the satisfaction that he gets in improving this famous game way beyond what the original

This seems to clearly be a passion project. The author is being paid in the satisfaction that he gets in improving this famous game way beyond what the original creators managed to do, all while being bound by the same technical limitations. He also gets to show off his skills to a fairly large (in absolute numbers at least) global audience of people who are also really into this particular game. I doubt you would see someone going to such great lengths for an obscure title that few other people would appreciate or care about.