That is only the game-logic (which has been 'open' since 1999) and not the engine. That is under a proprietary license and as per terms of the license incompatible with the code that the projects here are using (which are GPL, as they take code from the open-source Quake engines and community projects that have been created over the past 2 decades).

However the projects in question are outside of US jurisdiction for that to matter. They are in violation of id software's and Valve's copyright though.

You can find the original HL SDK license here https://pastebin.com/pAVKk1NL The copy on GitHub has just a find-replace of the Source Engine license, which is not compatible as well. Seems like Human Error either way.