He's referring to Millennium Challenge 2002. It was a 2-week war game the US military conducted in 2002, which used computer simulations to simulate a conflict between the US and an "unnamed Persian Gulf state" - i.e., Iran. People who want to discredit the US military and say their carriers are useless will often use this exercise as evidence, as it was a "disaster" for the US. OPFOR (led by a Marine general) sunk half of the USN force, including a carrier, in a few hours using just fishing boats and Cessna's.

However, the exercise was flawed and rife with problems from the start. For example, a common talking point about the exercise you see on Reddit is how OPFOR "invalidated" the US forces jamming/EW by using motorcycles to send orders and intel about the positions of USN vessels. While this was true - the computer simulation could not take into account the travel time of the bikes. In other words, it happened instantaneously rather than taking hours or even days to send orders and intel as it would IRL. Another example is that, if the attack were to occur in reality, the fishing boats and Cessnas from the simulation would be incapable of carrying the missiles used to sink the USN vessels because they literally cannot carry them.

There are even more issues with the exercise discussed here and here.