Hand ball law as it stands:
It is an offence if a player:
- deliberately touches the ball with their hand/arm, for example moving the hand/arm towards the ball
- touches the ball with their hand/arm when it has made their body unnaturally bigger. A player is considered to have made their body unnaturally bigger when the position of their hand/arm is not a consequence of, or justifiable by, the player’s body movement for that specific situation. By having their hand/arm in such a position, the player takes a risk of their hand/arm being hit by the ball and being penalised
Ruben's arm didn't make his body unnaturally bigger as his body movement was justifiable. Where else could he have naturally put it. He was turning around in mid air.
Still haven't seen an angle which shows Fabinho headed it onto his arm although from the only angle I have seen it appears that was the case in which case it's no penalty. That said didn't Spurs concede a ridiculous penalty last season or possibly the one before when an attacker headed the ball onto the defender's arm who wasn't even looking at the ball. VAR judged that to be a handball despite the ref not awarding the penalty at the time. And then of course on the other scale is their goal against us in the CL which was handball but the ref wasn't shown the correct angle. At least in rugby not only do we hear the conversation but the ref asks for every single angle available not just what the broadcaster wants to show.