As Referee's, we're not here to criticize and we all have different views and interpretations of the laws. Here we have an example of potentially where the referee was looking for a reason to match the 'incident' to a law.
If we view this in real time, it is late and potentially he could have changed angle or slowed quicker.
With the aid & luxury of tv replays we can see he doesn't try and stop but has just landed from an attempted charge down. At the point of impact he is turning which brings the arm/forearm into question. However there is not forward movement of the arm and this becomes more of a knock or nudge to us.
Our view, penalty and a talking to for being lazy and careless.
However the sanction from one of the best referees in the world of a red card does leave us scratching what hair we have left on our heads.
We are all about contextual judgement (read the post from Chris Assmus on this topic) but surely this was not applied here.