I'd assume they use 1000 distinct IPs because the system scaled to 1000 instances and presumably many of them came from related IPs. So it makes IP banning considerably harder, but not impossible.
More importantly, any reasonable IP-based approach would have a lot of false positives. What if there’s an RSS service like Feedly running on the same cloud?
Depending on your product, blacklisting all AWS IPs might be acceptable. For example my company has a VPN exit on AWS, which appears to be blacklisted by twitter.