[postfuturist]

"Similarly, if an employee smuggles in GPLv3 code in to the company's codebase then the company can't just shrug that off, and must release their proprietary source code as GPLv3."

No, assuming the company was in violation of the GPLv3, they would probably need to stop using it, and potentially pay damages if sued by copyright holders, but would be under no compulsion to release their own proprietary source code. Unless, of course, they wanted to comply with the terms of the license and continue using it. However, the GPLv3 alone wouldn't even require that unless they were selling or making available copies of the software.