You gotta by Homefront: The Revolution to get it, turns out there was more then just those two levels remade inside it. The whole game is complete but the devs forgot the code to access it, luckily along comes the modding community who find both the game and the codes to access it.

https://www.gamespot.com/articles/timesplitters-2-4k-port-hidden-in-homefront-the-revolution-discovered/1100-6490028/?utm_source=gamefaqs&utm_medium=partner&utm_content=news_module&utm_campaign=homepage

By the way, why hasn't the Homefront saga been released on GOG yet? Deep Silver could do us this favor.

Is it really time for us to... dig out the Xzibit memes?

They'd need to make GOG versions for these, as they are newer games.

These games would probably would need its Steamworks dependencies removed/commented out/or something; its DLC's able to work offline; Steam Achievements moved over to GOG Achievements; Steam Cloud support moved to GOG Galaxy Cloud; and it might be GOG Galaxy Multiplayer added to get the MP component going here.

Who is "they?"

Ever since people found out the easter egg, HTR's reviews are all "comes with a free game called Homefront: The Revolution". But I honestly don't understand why they put an amazing game inside a mediocre one. They could have just sold it separately...

Well that's weird, why would they even do that? Why not just release it separately?

Well I thought THQ Nordic have the rights to it now so there might be your issue to release it separately. :-P

hackers.

It runs pretty well, but the later levels tend to crash quite often, which is a shame :(

Pretty funny considering the huge number of fans asking for a remaster of this game.

Maybe THQ Nordic didn't think, for some reason, people would want and buy a 4K TimeSplitters Remaster...
...so Dambuster didn't care and just snuck it into another game anyways?

[shrug]