Yep I agree - wonder what the actual vis was that high up in the mist layer.. I think the only reason this doesn't happen more often is because of probabilities. We hit cars on the road because it's a 2D thing, with aviation being 3D the chance of 2 planes being in exactly the same piece of sky is fairly remote.. just luck wasn't with them today :S
The more airspace we get confined to sadly the more likely these things are gonna be :/