According to Techspot, "Half-Life 2 mod builds the game using the original’s engine ", known as Half-Life 2 Classic.
https://www.techspot.com/news/71029-half-life-2-mod-builds-game-using-original.html
I had to read that headline three times and only after reading the actual article did I understand what they were talking about.
Just a terrible title job. It should say “Modders demake Half-Life 2 using Half-Life 1 engine.”
Yeah because it’s not a Half-Life 2 mod if it uses the Hl1 engine. Then it’s a Hl1 engine mod about Hl2.
If I make a mod about Doom, it doesn’t become a Doom mod, it’s still going to be a HL2 mod. 
I checked the demo out last night. While I like the idea of recreating HL2 into HL1, and I’m impressed with some the effects pulled off in the mod, I’m not sure I’m crazy how the mod is being developed. The architecture and layout are very impressive, but I’m not crazy about the models. The best mod I’ve encountered which recreated HL2’s mood rather well in HL1 was Reissues, with the level “It’s safer here…” I’m impressed with what’s been done, but it definitely needs a lot of work, and a lot of alternative solutions. I have some ideas to improve it.
*Don’t use Source models. Just use classic NPCs from Half-Life, or classic NPCs with different clothes and altered skins. For the rest of the NPCs, like the Citizens, Breen, and Mossman, just create Half-Life 1 models and skins which approximately look like HL2’s NPCs.
*For the beginning scene, don’t try to emulate the cutscene nature of HL2, or use the Source G-Man skin. How about just paying homage to the end of Half-Life 1, where the G-Man teleports you to levels based off the visions you saw? It would begin with a black screen, your vision brightens up to see the G-Man in front of you in an empty space, and then he teleports you to the test chamber, then inside the Citadel, and then finally in the train, and then your vision flashes and he disappears.
*Combine Civil Protection needs work on its skins.
As for everything else, I have no clue how they’ll pull it off. There are so many things in HL2 that are hard to replicate, let alone make a clever approximation of, in HL1. Pulling off a gravity gun in a late 90s game is not impossible, since modders for Dark Forces II: Jedi Knight managed to pull off a gravity gun mod which worked. Barneys and Grunts could serve as good templates for Civil Protection and Combine Soldiers. I kind of want to see a variation of the Pit Worm to be used for the Strider.
What’s next? HL1 demake on Quake 2 engine? Quake 2 demake on Doom engine?
I found a gameplay video of the map if anybody else is interested how it looks like.
This idea was started on this very forum many years ago. Some folks started HalfLife2-gldsrc. They had a modDB page and everything before they realized they had no idea what they were doing or how to do it