It is entirely possible that it is not an intentional DoS attack but rather is a DoS from the non-malicious clients of SR. Hidden services scale horribly and once you get past 200 or so simultaneous users things really get sketchy in regards to reachability. I don't know why they have come to the conclusion that it is a DoS attack though. If it is naturally arising from Tor being incapable of coping with such a popular hidden service, one thing they could do is create a second .onion that points to the same thing. Freenode is a popular IRC network that recently had to do this because their original .onion had its introduction nodes DDoSed from all the legitimate users it had. Unfortunately it is really trivial for malicious attackers to make it impossible to reach a .onion though, I will spare the exact details of the attack though. This is really a design problem with Tor, and unfortunately it is not even very high up on the developers list of things to work on. Most of the funding and research going into Tor currently involves bypassing censorship attempts at the ISP level (ie: bridges, obfsproxy), hidden services are somewhat of an after thought, and the primary developer who was still maintaining hidden service code quit working for Tor Project quite a while ago.