Yeah, I don't know. The biggest Mirai traffic spike involved a pretty simply volumetric GRE attack; GRE is its own IP protocol, so I mean it's trivial to filter but also lots of middleboxes won't even forward it in the first place. There was some confusion about how bad the Mirai attack was because the propagation code for Mirai, independent of the DDoS attacks, managed to crash some routers.

It's definitely not the case that all DDoS attacks can be reliably cleaned up in an ISP scrubbing center.