What Fastly has done starting from Varnish 2.x was what they needed to do, to do what they wanted to do: Build a global CDN business.
There are difference between "running Varnish on your website", "running Varnish for your web-hotel customers" and "running Varnish for some of the biggest web-properties in the world" and software and organizations always mirror each other.
For instance Fastly does not run Varnish on Solaris, FreeBSD, NetBSD or ..., they run it on whatever version on Linux they decided on, and what ever brand and model of boxes they find optimal, and therefore they their own code and changes do not need to be portable and can be tuned with laser-like precision to their hardware and kernel they use.
Even if they threw their codebase up on github, I doubt very many organizations could or would use it, because it is chock full with interfaces to Fastlys business systems, encapsulates their network strategy, and God knows what. (Remember when Y! threw Inktomi over the fence ? Not terribly useful if you were not Y!, it took years to generalize it.)
And you will find the same situation, no matter which other major FOSS based organization you look at, Amazon, NetFlix, the pink-bit-pushers, FaceBook, Twitter, Google ...
You simply don't run a huge company on vanilla software, if nothing else because your geeks can not resist the temptation to improve and optimize it to ease their own jobs.
And just to stake the "Fastly vs. Varnish project" thing through the heart at the cross-roads: Fastly is a major sponsor of the Varnish Cache Project.
While much of the original comment was anecdotal the fact Fastly has extensively worked on custom subsystems for it is not wrong, correct?
Addressing why they had to (and the original comments speculation/accusation about that) would be helpful in clearing up any confusion.
The OP was absolutely too aggressive in his comment, so I understand why you’re very defensive.