That's in the section beginning, "deliberately touches the ball with their hand/arm", but it clearly isn't deliberate from Grealish
This is the bit you should focus on: "touches the ball with their hand/arm when it has made their body unnaturally bigger. A player is considered to have made their body unnaturally bigger when the position of their hand/arm is not a consequence of, or justifiable by, the player’s body movement for that specific situation.". All you need to do is watch Grealish jump to see that his arms are moving due to his desperate leap to head the ball. You can even see him try and move his hand back downwards
That the refs decided his movement was unnatural shows they've never jumped for a ball in their lives. Farcical decision.