Watching the video, I think he likely touched it as it went by. It looks like the spin changes slightly, but I really can't tell at this resolution. If that was my view in a game, I think I'd give the GK.
But the R has a different angle, and its different watching live where it may actually have been easier to see a slight change of direction.
For the GK, I see two options:
(1) He knew the law, but knew he touched it--tough as GK to fight that instinct in the heat of the moment.
(2) He didn't know the law at all--which is why he is looking at his teammate not the ref (or not berating himself for touching it)
Easier to assume the ref didn't know the relevant law rather than he did but saw a keeper touch I'd go with ref's call on keeper touch.
I wouldn't have given the free kick in the first place though. It was a physical but fair challenge for me. Can't see the ref or the AR but the delayed whistle and player reaction suggest the AR flagged it and the ref didn't want to throw him under the bus.