DENYING A GOAL OR AN OBVIOUS GOAL-SCORING OPPORTUNITY
Where a player denies the opposing team a goal or an obvious goal-scoring opportunity by a handball offence the player is sent off wherever the offence occurs.
Where a player commits an offence against an opponent within their own penalty area which denies an opponent an obvious goal-scoring opportunity and the referee awards a penalty kick, the offending player is cautioned if the offence was an attempt to play the ball; in all other circumstances (e.g. holding, pulling, pushing, no possibility to play the ball etc.) the offending player must be sent off.
A player, sent-off player, substitute or substituted player who enters the field of play without the required referee's permission and interferes with play or an opponent and denies the opposing team a goal or an obvious goal-scoring opportunity is guilty of a sending-off offence
The following must be considered:
distance between the offence and the goal
general direction of the play
likelihood of keeping or gaining control of the ball
location and number of defenders
The bit you've quoted isn't the relevant section. The first paragraph is about handball, the second is about penalty area offences, the third is about players entering the field of play. Dunk's was none of these.
The bit you want is just above:
SENDING-OFF OFFENCES
A player, substitute or substituted player who commits any of the following offences is sent off:
denying the opposing team a goal or an obvious goal-scoring opportunity by a handball offence (except a goalkeeper within their penalty area)
denying a goal or an obvious goal-scoring opportunity to an opponent whose overall movement is towards the offender's goal by an offence punishable by a free kick (unless as outlined below)
So - it's a red card if Dunk denies the opponent an 'obvious goal-scoring opportunity'. We all agree he did. But, if the attacker had stayed on his feet, and still had that goal scoring opportunity (whether he scored or not), it wouldn't have been denied him. Therefore, would not have been a sending off offence.