This is pretty wild! Seems like it's running the FWGS xash3d engine, which seems to be some sort of rewrite of the Valve goldsrc engine (which I didn't realize had available code to be rewritten)? I'm curious about how it's actually executing in the browser (WebGL? WebGPU? WASM, even?) but cant seem to be able to get onto a server to poke around at the moment.
The original Half Life, including goldsrc, is source-available: https://github.com/ValveSoftware/halflife
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.