I don't think this is about regular load balancing. DDoS is coming from a large number of infected machines, but they can't control how their traffic is routed. By using anycast you're splitting the machines that are used to attack into small groups that your pattern based mitigation or even your regular reverse proxies can handle.
CDN networks are well equipped for this because of their large geographical footprint. If they can terminate "bad" requests closer to their origin then they don't add up nearly as badly for the application server.
