https://github.com/FWGS/xash3d-fwgs
Easy to use Mac build here: https://www.macsourceports.com/game/halflife
Edit to answer myself: Looks like this is more of an offshoot of the FreeHL projects by the same author, which rewrite GoldSrc game logic to QuakeC to get those games to run on open source engine stacks, where the utility is more obvious. I guess it was just fun to see how hard it'd be to get HL2 content running.
A bit similar to the OpenMW project working on Oblivion and Skyrim content loading on the side, though perhaps that's a more obvious future vector for that project.
Funnily enough the looks of this HL2 through this engine makes it flow more with HL1 than I could expect; an interesting reverse Half Life: Source / Black Mesa / demake of sorts.
Seems to be using a dnsft.cloud.zyxel.com certificate. Is this a home router?
Then again maybe that level of detail even in idtech1 would have required more computing than was available for many years.
Good job keeping me away with Anubis, btw.