The current wording of the rule:
"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 offender is cautioned if the offence was an attempt to play the ball or a challenge for 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."
So he probably thought it wasn't an obvious goal-scoring opportunity - which is probably true as the ball was going out.
Although I have read that wording 3 times now, and I'm not exactly sure.