I believe it is thriving because it was well-designed, by extremely influential individuals, and the early library work was stellar. Also, several other experienced and influential programmers tried it, and expressed something along the lines of "programming is fun again!"
Inside Google, the two main programming languages are C++ and Java, not Go (at least when I left, in September). The Go tooling is generally less capable, but the interfaces smaller, and often nicer: they have the dual benefits of hindsight, and a small but very smart team that really cares about conciseness and clean APIs.
Of course, it's undeniable that the Google name helps a bit. And paying a team of very experienced (and presumably very expensive) developers to work on it makes a huge difference. But I think it would be as successful if those same developers were sponsored by Redhat, or Apple, or just about anyone.
Dart is also Google sponsored and no one uses it despite the fact that it's actually a pretty great general purpose language. People use go because it's productive and had a PHENOMENAL standard library for networking.
It clarifies the fact that Go is successful for more reasons than just being pushed by Google. So it focuses the question to "what is it that people like about it". And then we can have a better conversation.
Your theory fails to account for the lack of success with respect to Dart; so, it seems more like something you have an urge to believe (despite a lack of evidence).
Dart has been abandoned by Google the day that Angular team has chosen Typescript instead of believing in Dart, thus sending to the world the message that the company doesn't believe in it.
Whereas there are a few production examples of Go at Google.
My understanding is that Dart is used by Google Fiber for their routers, so I wouldn't call that abandoned yet. But, the point is that Google supporting a language does not seem to imply its eventual success.
If it came from "Joe the dev" no one at HN would give it a second look.