Exactly the law is clear,
The law says :
Challenging players in the air - Law 10.4(i)
Play on – Fair challenge with both players in a realistic position to catch the ball. Even if the player(s) land(s) dangerously, play on
Penalty only – Fair challenge with wrong timing - No pulling down
Yellow card – Not a fair challenge, there is no contest and the player is pulled down landing on his back or side
Red card – It’s not a fair challenge with no contest, whilst being a reckless or deliberate foul play action and the player lands in a dangerous position
Most of the a time one handed slap/tip, does still have the player in a position "to catch the ball" should they wish to, that is fair enough, you have to take the whole move in context, and in this case there was zero way Read had any chance of catching the ball, therefore it's a penalty instantly in my book, as it was not a "fair challenge" as Read was never in a "realistic position to catch the ball"
I ref a lot of matches and i stick to the letter of the law, it's a pity international ref's can't as well.
Just because they might have got away with it previously, or if they do it routinely in NZ rugby, does not make it an entirely legitimate play.