Actually, in this case not at all. First Microsoft and Google are two companies that heavily support and participate in FOS. So just because something might be under Microsoft or Google wouldn’t mean that the source code wouldn’t be available to be seen by anyone.

Beyond that, this issue wasn’t actually discovered by looking at the code. This issue was discovered because of a performance problem. So, even if the problem had been in proprietary code, this could have been easily reported as a bug. It could still have been discovered and fixed in a regular release cycle.