I managed to claim 64 out of 256 blocks using proxies from Bright Data[0] and PacketStream[1]. I claimed 49616 IP addresses within those 64 blocks. Unfortunately, the website doesn't tell you how many IP addresses someone claimed in total. Cool project!
The attack discussed in the blog post was an HTTP attack, so reflection and amplification were not involved. The attacking machines are most likely infected, and can be labeled and tracked.