Alonso's car was rotating about a vertical axis such that his front right wheel was coming back, relatively, towards Leclerc's head. See the front right tyre's location here:
Witness mark on halo and top of tub. Without the halo, it's more than likely that the tyre would have gone across the top of the cockpit. Would it have hit the helmet? Who knows. But with the halo there it absolutely didn't hit his helmet. That chalks up one for the halo in my opinion.
Interestingly, Alonso's car cleared Leclerc's roll hoop - the camera is in place and intact (on board video continued after the impact.
If you are more fortunate than others, build a larger table not a taller fence.