It’s not that they got DDoSed, it’s that unregulated off-shore gambling is illegal in many countries, so their IP addresses were getting blocked in these countries. The way CDNs like CloudFlare work is that many customers share the IP addresses, so they were getting other CloudFlare customers blocked as well.

CF wanted them to move to a “bring your own IP” plan so that their IP blocks wouldn’t affect other customers, and that came with the steep price tag.