The handball law is due to change (for everyone) next season and UEFA/FIFA like to try things out in the upcoming season in the prestigious tournaments (albeit unofficially).
https://www.bbc.co.uk/sport/football/47429316The fundamental change is this one:-
Another change to the laws of the game means that if the player's arms extend beyond a "natural silhouette", handball will be given, even if it is perceived as accidental.
Under those rules it would have been a penalty. The ref would have decided that Kimpembe's arm position was outside the "natural silhouette" (see pic below) and so, even though it was not intentional, it would have been given. Turning his back has nothing to do with it in this case, nor the amount of time he had to react.
Under the current rules it's no where near as clear cut. It definitely hit the players arm, and the contact was definitely in the penalty area.
Currently the only get out is whether it was unintentional (or, more correctly[1], could Kimpembe have got his arm out of the way in time) but that only applies where the ball is struck from a yard or two away and the player has no chance of getting their arm out of the way. This was just a bit further than that. If the ball hits your arm and you've had 10 seconds to get your arm out of the way then you're deemed to have deliberately left it there. Obviously this is somewhere in between the two extremes. Even if Kimpembe had been looking he wouldn't have had much time to get his arm out of the way of Dalot's shot, but if he had he might have been able to and the ball might have sailed over for a goal kick.
Given there's no unanimous agreement amongst referees it shows that the laws are still open to too much interpretation, however IFAB are making changes (as noted above) to slowly try and solve this.
Here are the pics that show Kimpembe's arm is not in a standard position, and the position relative to the penalty area:-
1. The "deliberate act" the law requires can be interpreted as the deliberate decision not to move the hands/arms out of the way. You don't have to have movement of the hand/arms towards the ball for it to be handball.