TFA claims that Anycast is an advantage when dealing with DDoS because it helps spread the load? A regional DDoS (where it consistently hits a small set of DCs) is not a common scenario, I guess?

Basically yes. Large-scale DDoS attacks rely on compromising random servers and devices, either directly with malware or indirectly with reflection attacks. Those hosts aren't all going to be located in the same place.

An attacker could choose to only compromise devices located near a particular data center, but that would really reduce the amount of traffic they could generate, and also other data centers would stay online and serve requests from users in other places.