You were getting 1k rps, and each request was from an unique IP? So after an hour you got hit by 3.6M different IPs? And all from uncorrelated /16s? That seems hard to believe. Not that I don't believe you, it's just hard for me to grasp that whoever was scraping you had such a large and distributed swarm.
This is called rotating residential proxy service. You can buy it off grey market sites that are probably getting it from botnet operators. It costs about $2-$5 per GB.
