Note that "the auth servers hosting myetherwallet.com" aren't some random IP addresses in Tallinn, Estonia. It's Route53!
I get what you're saying with your counterfactual, that DNSSEC "breaks the exploit", requiring the attackers to use a slightly different exploit. But who cares? The slightly different exploit has the same predicates.
That's what's so funny about the narrative that DNSSEC has some role to play in today's attack. It would almost make sense if the narrative was "we could use DANE to get rid of all CAs and then it wouldn't matter if you could trick a DV validator". But, no, people are talking about how DNSSEC would help LetsEncrypt, in the event of a global BGP hijacking. Admit it! That's funny.
I think he's saying: let's say the correct IP address for example.com is 192.0.2.80. Instead of hijacking the prefix containing example.com's nameservers, an attacker could just hijack 192.0.2.0/24 and immediately get a DV cert. Within seconds they would be up and running and DNSSEC wouldn't have done a thing to prevent it.
I posted in response to pfg's comment above a mechanism that high value targets could choose, which is modelled on how Facebook behave today but (since we're proving a point here) more extreme. Make a deal with a CA, forbid all other CAs from issuing.
The other thing about Let's Encrypt is that they wouldn't be a good target for such an attacker because they are seeing the world from more than one place. You specify particularly a global BGP hijack, but those are hard, which is why this wasn't global. Tricking just, say, the original San Francisco office of Let's Encrypt doesn't get the job done.
If myetherwallet.com had been DNSSEC signed then users who validated the signature chain would have not been redirected to the fake site. But it's not DNSSEC signed.