I'm guessing cyclic redundancy check. You turn the game state in each tick into a number, then play the exact same game on both platforms. If you get the same stream of numbers, you can be pretty sure the game is behaving correctly.
It's likely a hash of the game state. They save input and playback recordings of the game, expecting to always receive the same hash for the game state every frame.
Just this week I started on a similar system for a game engine I work on (Zelda Classic). Main reason was for testing purposes, but another benefit is that it is SO cool to see the game replayed for you. Like those self playing pianos.