This isn't even true. The Wickersham Wall, as it's known, has been summited by a few different parties. It is true that the Harvard route that Graham's party took has only been ascended once, but the Canadian route has been successfully climbed by several parties. And it's not that the Canadian route is easier or anything, in fact it's certainly the more technical of the two. It also happens to not share the Harvard route's high level of avalanche risk. And, lest anybody get the impression that this is some highly technical route, keep in mind that it has been skied.

So basically Graham was on a team that was lucky enough to not get hit by an avalanche on a route that other climbers are smart enough or technically proficient enough to avoid.