This is seriously the stupidest thing I've heard in months and clearly comes from someone that knows nothing about the basics of an OS or security. Windows is literally MILLIONS of lines of code and requires a massive undertaking to design and interface with all the hardware options out there. Standards development alone takes a ridiculously large and coordinated effort. Not only that, for any practical purpose so many engineers and others have to know about the specifics and intricacies of that software to make it work, the obscurity will be next to worthless. Even now, as mature as the Windows codebase is it still takes a pretty huge team to work on it, and their security is definitely not top notch.
Now imagine having to employ a massive division dedicated to nothing but that within the military. It is government, so top tier talent won't go there on principle and pay will be mediocre at best compared to private sector, plus any military personnel working on it could end up just straight vanishing when their enlistment ends. So now you need a top notch, reliable, full featured, high security OS developed by middle of the road developers that works with a variety of hardware that is not standardized necessarily to be optimal for your OS. Sound like a nightmare to you yet? Not only that, as someone that has done a lot of reverse engineering in my time, it takes a MUCH smaller effort to reverse engineer systems like that then it does to build them. Then once the inevitable unlocking of the base gates happens that everything is based on, what do they do? Design another OS and play the most expensive game of whack-a-mole in history?
If I am a military leader in the cyber operations side of the US military, I'm fucking ecstatic that the Chinese are wanting to do this. They will waste massive amounts of resources trying to do develop it, create an absolute logistical nightmare for themselves to deploy it, make it easier for US intel to break into it, and make it easier to not affect our own stuff with introducing any 'backdoors'. All the while the US can simply work with Microsoft to harden our existing infrastructure and build on decades of work and lessons learned.
There is a reason that there are only a few bases of large scale OSs (not counting embedded systems, those are a different beast).