So for some backstory:
Homefront: The Revolution is the second game in the Homefront series, being a "sequel, but not really" sort of game as it explores the same story idea as the original (taking place in an alternate history where North Korea actually managed to take over much of the world, and is in the process of taking over the US, though guerrilla fighters are fighting back), but isn't actually a followup to the first game (the original was basically a CoD clone, while the sequel is more akin to something like Far Cry). In the game a fun easter egg that was placed in it are arcade cabinets which let players play through two levels from the classic sixth generation FPS Timesplitters 2, as the devs behind Homefront: The Revolution, Dambuster Studios, used to be Free Radical, the creators of Timesplitters (and TS itself is a spiritual successor to Goldeneye on the N64 as it was made by many former Rare devs who worked on that game). People discovered back in 2016 when the game released that assets for the entirety of TS2 are in Homefront's game data, but it was unknown if a method existed that would let people access more than the two levels that can already be played.
On Easter this year a thread was started on Twitter where game devs were encouraged to talk about fun easter eggs they put in games they worked on. A designer who worked on Homefront stated that they actually did in fact put a cheat code in the game that would allow access to the full story and arcade modes of Timesplitters 2, but confessed he couldn't remember the code as he had lost the notebook with the codes for all three versions of the game, and the one friend of his whom he leaked the codes to 5 years ago released the codes on a Discord group but said friend got banned because the moderators of this specific Discord server thought he was lying. Now that people knew for sure a cheat code exists got unlock TS2 in Homefront data miners began to look through the game again, and rather quickly they managed to find the cheat codes for all three versions of the game.
So for anyone out there who owns Homefront: The Revolution on either PS4, Xbox One, or PC, here are the codes for unlocking Timesplitters 2 in the game: