Nibb31 - - that is as close as the other techies I have heard speak of Microsoft / I have heard some talk that Win 10 was untested until after it had been released to the general public to use, then it was hired out to a 3rd party company to fix all the problems as they arose.
To go along with the O.P. @phoenix
But now after Microsoft's announcement even Windows will now include the same Linux Kernel that all Linux OS use or will be using in the future. If you prefer a simple search will yield plenty of info about it.
Links about Windows 10 and Full Linux
https://www.theverge.com/2019/5/6/18534687/microsoft-windows-10-linux-kernel-feature
https://devblogs.microsoft.com/commandline/announcing-wsl-2/
https://www.digitaltrends.com/computing/windows-10-will-include-a-linux-kernel-starting-with-summer-2019-insider-builds/
As for testing the Linux Kernel and security issues all Android Phones run on a LTS Linux Kernel as well.
A search will give a ton of links talking about it, even articles by the guy "Linus Torvalds" that is given credit for starting Linux spoke on the topic.
Don't forget all the servers that run on Linux as well ........ as does Google and Facebook run on Linux.
If I was to just guess I would say it would be over 1 billion items run on Linux daily / I am counting Cell Phones, Notebooks, all computing devices and Tablets, commercial servers running it also, oh all most forgot Chrome OS and all of it's devices run on a Linux also.
I would guess world wide it is getting more testing on a daily basis then any other OS in my estimation.