It's hard to know until/unless they are fully transparent. It was almost certainly inside plant resource exhaustion, otherwise you wouldn't need an IoT botnet to cause it.

Multisourcing gives you at least path diversity in this case. Hopefully application software performance as well.

This is handled in the Verisign example by basic mechanical sympathy; when your machines can push 10gbps (probably 4x that in the years since the talk), it is economically silly not to simply eat the DDOS.

Eating a DDOS is always the first mark of maturity. You can then improve with additional network intelligence. The most common way is with using sample based packet analysis. You match an attack signature and feed that into your gear at the peering points using BGP with blackhole destination. You are now filtering at ASIC speed, terabits per second on modern chassis switch or router.

I re-read your initial comment and apparently I misinterpreted what you wrote, so thanks for the high-quality answer despite this. (You didn't explicitly call out Dyn for not double-sourcing, but their customers, and only used it as an example for good practices in the Verisign example. Since you mentioned it lot, I gave it to much weight and read it as "Dyn failed because they didn't double-source", for which I see no evidence.)