Under 30 is the key here. Think of all the people — who are OVER 30 who have never heard of Reddit — just as they’d never heard of Twitter or Facebook. The point isn’t that this will make the site catapult in popularity — as you note, it’s already quite popular (though it’s Alexa ranking is suspiciously low — too low) already, the point is that it’s now something that we will start to see reference in the mainstream — think your parent’s media — which hasn’t been the case before.
Even in the SOPA coverage, you almost never heard “Reddit” mentioned on-air or on radio. I know because I was watching for that moment – it didn’t come. This changes that.