How does he know? Think about how a game works.
Data is pulled from the harddrive and loaded into your ram. How fast this happens depends on three things, the speed of your harddrive(s), CPU, and Ram. If you don't have enough ram, then setting up AID0 will help, because when you need to pull info from the array, it will come quickly. Its also a dumb idea to do this, as increasing your ram will help much more.
For many games, once its loaded into ram, thats it. (MMOs are the obvious example of games then need to go back to the drive.) A FPS game shouldn't need to go back to the drives until its done with that level, and needs to load the next one. It doesn't matter if your drives are lightening fast, once the level is loaded, its running from your ram, not your drives.
As was mentioned, games can use compressed textures, that need to be decompressed. Info also needs to be feed to and from the RAM and video card. This is a CPU load, and has little to do with your harddrives. This is why synthetic apps show huge gains for AID0, while real world apps tend to be much smaller.
If you want more info, start reading Tom's and Anand's harddrive reviews. Make sure to look at real world game testing, not PCmarks and 3DMarks.