It would require thousands if not tens of thousands of Google semployees coordinating in utter secrecy

This is usually used for things like the Moon Landing, where so many folks worked for NASA to make it entirely impossible that the landing was faked.

But it doesn’t really apply here. We know for example that NSA backdoors exist in Windows. Were those a concerted effort by MS employees? Does everyone working on the project have access to every part of the code?

It just isn’t how development works at this scale.