"© 2013 HIDDEN VALLEY TEAM. VALVE, STEAM AND THE STEAM LOGO, SOURCE ENGINE ARE TRADEMARKS OR/AND REGISTERED TRADEMARKS OF VALVE CORPORATION" ?
I don't think it says the dev team itself belongs to valve. It's probably because they're using the Source Engine and their game or something.
Nigga, did you even read?
"HIDDEN VALLEY TEAM [...] ARE TRADEMARKS OR/AND REGISTERED TRADEMARKS OF VALVE CORPORATION"
From reading some more through the Steam Community page, turns out they're a Russian/Ukrainian dev team who do this with no budget. But everything in the Greenlight page points to VALVe being the owners of the studio.