You don't have to track literal records, though, if you can nail down a specific line of descent. For nobility in particular, if you can firm up ancestry up to X date, it's just a matter of working backward. And noble trees are easier to track than non-noble ones. The outlandish thing isn't claiming descent from Robert the Bruce (or any comparable figure), it's treating it like a Big Deal when it isn't, because, as you say, it's extremely widespread to the point of being meaningless.