Having poked around in this space before on the Vectrex and 2600 (and having left because of being harassed by guys like bigh4th), let me tell you... what Jeff has done is no small achievement. It's not only coding in a 4k space (though I would bet a DK board has more than that), it's coding in assembly on a 25 year old CPU. There aren't exactly a wealth of resources out there on 25 year old assembly languages and there aren't that many people anymore with actual professional experience on them - which means if he's not one of a rapidly shrinking group, he had to extend a modern programming education back into far more primitive techniques and spaces - stuff they just don't teach in college anymore unless you look hard for it. I'd love to hear from the guy how he went about it - whether he reproduced from scratch in assembly or if he located a disassembler, analyzed the results, and then went from there, as well as where he managed to find that compiler.
Since he's writing on a closed platform, the question would be does he owe any license fees to the copyright holders of that board layout. Since there never was a licensing arrangement, and since it has been 25 years, I'd say no. That leaves the two remaining issues of IP: is he recycling pieces of Nintendo's original code and is he reproducing actual visual IP? Both are possible, and not having seen this game I can't give a reasonable guess, but again... it's 25 years later and not being commercially produced. There just isn't any profit being made, and therefore, it isn't worth the single hour a Namco retained attorney would spend drafting a C+D.... unless they decide to use this as demonstration of IP protection. That could happen. In trademark law, if you don't defend your trademarks, you lose them, even if the violators are hobbyists like this guy. If he's going to have a problem that is where it will happen.
I love that people are still out there doing this and I really, really, really hope some bonehead with a poor understanding of hobby doesn't come along and dump these out for MAME.