9"/23cm hail would do you in pretty quick I would think. Even using my entire forearms I can only manage to partially cover my whole head. You'd be better off dropping down into the fetal position but if all the injuries were to the top of the head then it sounds like they didn't do that (maybe it started too quickly).
These people were travellers, not joggers. They likely would have had some items with which to shield their heads - packs, pans, spare heavy clothing. Perhaps they did, and there were more than 200 - the survivors making it of the pass.
I think the comments to the article draw the correct conclusion - the author mixed up circumference with diameter, and the hail would have been 'only' 3 inches. Still pretty deadly, but not quite as terrifying as being bombarded with rock-hard soccer balls.
