to adding anti-cheat systems to the code for each game itself, i.e. if it were a top-down shooter, before the score is submitted, the game would check number/type of enemies killed vs. shots fired vs. score vs. a random seed per play to make sure that all four correlate before a score is submitted. Just an example.
Wont work. At the point the score is submitted to the server there is an HTTP call. Hackers can find that call, replicate it and submit it outside of the game with whatever value they like.