It's safe to assume that Windows is backdoored, as it's closed, not audiatable and is a part of PRISM. It's not a proof, but this is how good opsec works.

And yes, this is not equivalent to what the parent said, but your reaction should be similar.

While it is closed, we've had enough leaks from that era of Windows (which are still on GH) to know the kernel is not backdoor'ed.