Why? It is absolutely correct.
I run one website that is nowhere near the size of BBC, and it doesn't need its google traffic. All search engines combined provide less than half the traffic. In January there were 97 links that each provided over 100 visitors. Another couple hundred that provided over 50 each.
Well, you aparently need to hear it again, because it hasn't sunk in.
I know of one website that is in an extremely competitive field that gets the majority of their sales from links in their newsletter or repeat sales.
They also give out branded sports bottles and gym bags to their customers to encourage word-of-mouth advertising.
Some months they rank well and pick up new customers that way, some months they don't feel a need to chase down new customers, and some months they buy ads.
Have *you* considered doing anything to diversify your traffic sources?