"if google is largest proprietary software developer, which company is not?"
is "all the other ones".
I promise I'm not being purposefully obtuse, I really can't find any other meaning to that comment.
I really don't know how Google "ranks" w.r.t OSS contributions.
Company XYZ is the largest developer of proprietary software
Company XYZ is the largest contributor to open source
If we're talking about raw amounts, not %-of-company's-output