Is it? I thought GitHub, at least, was split Dyn/Route53 shortly after the Dyn outage started as a means of getting back online. Now, they've removed their Dyn, and are now exclusively Route53.

I don't think Dyn showed any incompetence; the parent-poster was merely remarking on relying entirely on a single provider, who, if they get DDoS'd, causes your site goes down. (There was some previous discussion about splitting between providers, but some commenters noted that it was difficult, or at least non-trivial, to replicate records between two providers.)