This is an engine re-implementation. That means that someone sat down and reverse engineered what the original game is doing behind the scenes and then rewrote it as they saw fit. It is legal and in this case the source code is there for everyone to see and edit. There are many benefits to doing this, most importantly preserving the software for new platforms and allowing it to run on systems and platforms it was not originally intended for.
That being said, it still requires the original data from the disc. There are instructions on the github page specifying exactly what you'll need.

Thanks