The small size of Valve is simultaneously mind boggling but also not, given its very intentional independence. I would have to imagine that they must contract out or have partners at least for their hardware relationships if not for their massively multiplayer online games. At just 350 people that's enough annual revenue to make everyone there a millionaire several times over. Simultaneously plausible but mind boggling.

It's well-known that most of the work on SteamOS is done by vendors on behalf of Valve (both individual kernel authors and agencies like Igalia).

They contract out all the time, they've admitted to it in lots of interviews. So I think through the amount of contracting they're able to keep their core hires down.