How so? As the blog post points out, the whole thing is open-source with almost all discussion surrounding the project happening out in the open on their GitHub repo. You might not agree with the direction the project is going, but I'd say it's about as well-lit as any open source project can be.
Having it up on GitHub after that doesn't really mean much. This is the same for Android and Chrome, of course.
A project run by a "benevolent dictator for life" (like Python or Elm) is entirely compatible with open source. The same is true for a project run by a corporation (like React).
With the main answer to questions being, we can discuss this somewhere else, some-other (unknown) time in the future, usually in a more private setting. Unless pressed further.
Case in point: https://github.com/ampproject/amphtml/issues/13457#issuecomm...
Or as carmforce(Ubl) so nicely put it. "our project, our rules" but those rules keep changing, are not public and the raised issue about defining/clearing up those rules is obviously not a priority.
https://github.com/ampproject/amphtml/issues/13597#issuecomm...
Yeah, well... "Open-source" in the sense that they throw something over the wall from time to time. "Almost all discussion" in the sense that they use a heavily-moderated public newsgroup for some things. It's better than completely internal development, I guess, but don't kid yourself: it's a PR exercise coupled to a near-complete capture of the RFC process.