Campo Santo, the minds behind Firewatch, are now a Valve-owned Studio. In a news post on the team's own blog, they broke the story themselves.
In Valve we found a group of folks who, to their core, feel the same way about the work that they do (this, you may be surprised to learn, doesn’t happen every day). In us, they found a group with unique experience and valuable, diverse perspectives. It quickly became an obvious match.
It's a lot nicer way of saying, "Valve gave us money and stability to keep doing what we're doing... plus, it's freaking VALVE, people." Think about it from an Indie Dev's perspective. If you were approached by a massive company like Valve, given the option to keep working on your existing projects, and essentially told that you and your team will be taken care of, wouldn't you take it?
The blog continues:
Yes, we’re still making In the Valley of Gods (as a Valve game!); yes, we’ll still support Firewatch; and yes, we’ll still produce The Quarterly Review and our regular blog content. Thanks so much for your interest in our games and we’ll see you in Washington. Cheers.