Are you sure about that? I don't think the number of different IP addresses matters, it's how much traffic the servers behind that IP address can handle. A typical large website will usually have one IP address but that goes to a load balancer which distributes the requests to any number of servers. The more servers, the more traffic it can handle.

I would guess that SteemIt.com can handle much more traffic than a single witness node so I guess the question is how much traffic does it take to bring down a single witness node and then from that you can see if it's reasonably possible to bring down 100 of them.