This is not "emulation", he's re-writing the engine in C++ using the original ROMs' data as input. Of course, by reverse-engineering the machine code to the required level of detail, plenty of bugs in the original game are found. Very impressive.
Rewriting games and applications based on a disassembly is an impressive feat, considering the amount of hacks which end up in production code.<p>Similar projects i can think of off the top of my head:<p>OpenDune, OpenTTD, ScummVM, OpenTyrian