That's not what I see. The contact that makes him fall is outside the box. He's already falling outside the box.
But there is contact that would reasonably impede a player that continues into the box (Tavares pretty much sitting on Elanga at the end of it). If you want to call it two separate infringements or one continuous is up to you, but the outcome is a penalty irrespective of which one you choose. If he had pushed Elanga outside the box so that he lost his balance and the kicked him inside the box then surely the kick would have been punishable too? Otherwise you could just foul someone outside the box and then when they fall you can do whatever you want with them even if it constitutes a foul, yeah?
Regardless of whether you think that particular incident is a foul or not, it’s pretty obvious and suspect that that foul, the handball penalty on Cedric where he is crawling and scooping the ball out of the box, the first potential foul on Elanga and Xhaka’s goal were given much less attention than when Arsenal had a VAR incident that could possibly go for them which resulted in a three minute review. It’s almost as if the clear and obvious bar became lower because the goal that resulted was offside, as if the offside is something that the referee should compensate for. That should be the story here, not whether one specific foul was inside or outside the box. It seemed like they were using every little opportunity to call clear and obvious in arsenals favour, and the arbitrarity (is that a word?) with which the guys sitting hidden away in the VAR room enforce that threshold is bordering on corruption.
Edit: I forgot the two-minute VAR review on the penalty we actually got. I mean, why the hell would you spend so long on the most obvious penalty ever if not to try to find something to overturn it? I mean, as soon as you saw that the ball hit his hand in that position above the head it should’ve been obvious that the clear and obvious threshold would never be met, yet we got, what, 10-15 replays of it?