I haven't look at this game in particular, but usually you need to buy the original game to get the assests (art, models, audio, etc).

Even official open-source releases like the Quake series still require you to buy the original game in order to run them.

They reimplemented the game from scratch, the whole engine, UI everything. However, the artwork involved is still something you need to get from the original game, much like that used to be the case for OpenTTD.