As Dan said, the React blog is where they formally announce things related to React releases and important things the community really ought to know about now. If you follow them on Twitter, you can get insights into what they're working on, and if you don't follow them... well, important comments will still bubble up to other sites like Reddit and HN (as this thread proves), and even if you don't see them here, the critical stuff will be posted on the blog or the official @ReactJS Twitter account when the React team is really ready to officially announce that info in its final form.
I see no reason why they (or any other developer of any kind with a personal Twitter account) should be restricted in what they can talk about. So yes, I would completely say that personal Twitter accounts don't have to be "high signal to noise". A Twitter account is what the owner wants it to be.
