To be fair it's not true. Hard courts have different type of speed (Cincinnati vs Miami masters for example). Grass unlike other surfaces (clay, hard) has unpredictable bounce and because of that you can make the argument that it's surface that deviates from others the most.
But I think clay and grass differ from hard equally. Sliding, movement, high bounce and slow speed on clay vs unpredictable slow bounce, fast speed on grass. And movement plays role on grass too. Watch how well prime Federer moved on grass and how some other players move much worse on grass than on hard.