Neteng here: without seeing the traffic charts for individual interfaces and aggregations, I would bet cloudflare has a shitload of excess inbound capacity. They are a content pushing CDN. I would bet that at a major IX where they have one 100Gbps port that their out:in ratio is 90:10 or greater. So if they only have 6-9 Gbps of traffic inbound on a 100GbE port to a fabric consisting of 80+ bgp peers, they have a lot of extra capacity to absorb unwanted inbound traffic before it becomes an operational concern.

Have seen edge traffic charts for major porn hosting companies and the out:in traffic ratio is like 97:3