You need to also consider the one a few minutes earlier that went to VAR review but wasn't given. For me, in that case, he was actively straining to get his arm out the way - short of some impromptu on-field surgery, there wasn't much more he could have done to avoid contact with the ball!
With the one that was given, it simply looked a lot more like a "neutral" arm position to me. I don't think he was deliberately moving his arm towards the ball, but he also wasn't particularly straining to avoid contact either. The slow-motions make it look worse, as it implies he had more time to react that he actually did - at full speed, it just looks like it came at him quickly and he wasn't able to avoid it.