That repository is not the engine, it's just the code for the client.dll/server.dll binaries that contain game-specific logic and some other tools.
https://github.com/ValveSoftware/halflife on the other hand is the full Half-Life (GoldSrc) engine, but it's not quite an open license.
https://github.com/ValveSoftware/halflife on the other hand is the full Half-Life (GoldSrc) engine, but it's not quite an open license.