It is not advanced technology: TCP just has no protection here. Anyone capable of in-path packet surveillance and in-/by-path packet injection on a significant link can pull off this exact same attack. You could co-opt a router to do it: GCHQ have.
We're going to need pervasive (authenticated) encryption to defeat it.
This, by contrast, is a widely-targeted, fairly dumb DoS payload - but of course, not every DDoS has to be smart! Scale does all the work, and dropping malware, albeit relatively benign malware, en masse like this yields a lot of scale. This is particularly bad when there are potentially more personnel adapting it to evade defenses than there are personnel trying to defend against it: bravo to the GitHub security team!