Looks like this may have been based on the nillerusr Source engine, which is based on a 2020 leak of the Source sources. nillerusr's engine has also been used to port some Valve games to Android.
Source 1 is a good example here, where licensees of source need to also obtain a havok license before Valve can give them any code.
https://developer.valvesoftware.com/wiki/List_of_third-party...
I wonder why it never went mainstream. The Unreal engine went on to power a massive number of games. The Half-Life engines birthed a lot of mods, but wasn't used by many full games.