[gclarkii]

If I had to guess, they notified the networks in question and either received no reply, invalid reply(f*@k off) or received a reply but no corrective action.

It looks like the "other cloud providers" corrected their problems, or something. Since Cloudflare was able to generate a usable signature, they could have used it in their routers to blackhole the traffic. Guessing here... It's been years (late 90's/early 2000's) since I've had to deal with enterprise networking.