If the tiny "GET" requests are flooding in at 1Tbps, then the responses are going to be orders of magnitude larger, maybe somewhere around 500Tbps

You'd still be filling the pipe. And possibly filling many small(er) pipes at interconnection points. The last meters of pipes into a rack isn't where the issues are.

I'm curious, what sort of datacenter racks do you run that can serve 665 Gbps of traffic? For instance, Google's ToR switches (in its own datacenters) as of Jupiter (2012) were 16x40G, which is just 640 Gbps. Obviously, all of Google can serve a lot more than that, and you can get absurd stuff like 640-port Infiniband etc., but you seem to have pretty unrealistic ideas about network capacity. And, as another comment pointed out, that's just the input for this attack... the output would be a lot larger.