Great question! I really have no explanation for you on that one.
Every user has it’s own client. When you “log on” to SAFEnet you actually request your own personal data-atlas. So if your username is “Username” and your PIN is “4726” A single Hash is made of both of these values. That Hash points to your personal file. When your client get’s the files (probably 3 Chunks in the network) it will provide your client with your personal file. A password will unlock it for you. That private file contains all your private keys etc. And all the links to your personal files. So you don’t log on to a server, you log on to the network by getting your own file. So there’s no server to be DDoS-ed. You don’t use a website to log on, but a client that you need to download.
This is a really great read. Worth every minute.