I was guessing the pirate bay which has been down repeatedly lately according to the headlines here

Nope. Controversial site is advocating for Chechen independence. Allegedly linked to extremist activities.

In light of Holder v. Humanitarian Law Project what is CloudFlare's argument that you are not "providing material support to terrorists"?

The article outlines Cloudflare being questioned about terrorist activity. So, unless I'm mistaken, that would exclude 4chan.