NewsBlur will check to see if it has seen the url before. If it has, then it redirects to the feed the url points to. There's a whole lot of dupe urls that point to feeds, which is what is happening here. As for the old articles, it just took a recent subscriber to load the feed.
I'm still sub'd to gizmodo and lifehacker, so they didn't go away. You were probably on a smaller feed that got its url cut.