I understood the original comment about EC2 as a mitigation strategy so the hosting company's infrastructure doesn't receive the brunt of the DDoS gigabits (EC2 will) and it can still service all their other clients. In that case, even if the DDoS'd site will still be flooded and unavailable, its hosting plan doesn't need to be canceled outright. I suppose they could just skip EC2 and null-terminate the DNS until the DDoS stops, but I don't know if this has further implications.

In any case, thank you for a very informative post.