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.
It's complicated further because the witness nodes IP's aren't always even publicly accessible, and they can be connected through VPN's to redundant nodes all over the world (example).
These attacks will only make us stronger ;)
Thanks, that post you linked to was really helpful!