Seems like you're splitting semantic hairs.
The language maintainers regularly and consistently use "golang" as a disambiguation:
* golang.org
* github.com/golang
* Twitter @golang and #golang
* golang-nuts
In this context, it seems pretty "official" and the rebuke "factually incorrect" or at least the rebuke applies equally to the maintainers.
That said, it was a single wayward comment and it is (in my experience) out of character. I want to acknowledge that it was rude and validate the person who was wrongly rebuked; I explicitly don't want to pick on anyone nor tempt the Internet to pile on.
Rob Pike corrects an audience question when the guy uses golang instead of Go.
"Sydney Golang Meetup - Rob Pike - Go 2 Draft Specifications"
I don't think "factually incorrect" is warranted. It's factually correct that the language is called Go. And while "golang" is a useful alternative for searchability and where "go" is taken, it is still a valid criticism to request using "Go" in natural language and prose. As for "official disambiguation", I'd say the most official thing said on the topic is this FAQ entry, which is quite clear on the matter: https://tip.golang.org/doc/faq#go_or_golang
(not saying Brad's comment isn't abrasive or rude. But it's not "factually incorrect")