Given that Revolv's server software was presumably running before Nest acquired them, and from all rumors, not improved since, what possible proprietary Nest code could it have?
Rather it's the third-party proprietary stuff. Someone else owns proprietary code you deeply rely on but doesn't share any of your business goals and needs. And when dealing with hardware, there is a whole raft of firmware and drivers that could be licensed this way.
Maybe it runs on Oracle. Maybe it has third-party stuff licensed for their use but not open source distribution.