Misleading. It's all about source files being held in a git repository, not open source technologies being used by the OS itself.
Actually not, since it's saying the OS is built (created/compiled) by open source software, not that the open source soft is built into the OS .
It's saying Microsoft use GIT to store/manage there internal code repositorys for Windows (and quite probably other products such as Office). Lots of companies use a Git repository (either public like github or internally self installed). No biggie. I don't see why this is news worthy? Whats next? An article that says some staff use cars to drive to work? Desks to put there computers on, ....
The difference here is that they're included with every OS installation and that's what makes those apps a 'component' of the OS and no different to other components such as Command Prompt, Microsoft Edge, Internet Explorer, Notepad, Paint etc... It's a completely different story if those apps were not shipped with Windows and you had to download them. Apps included with the Operating System have different requirements and including 3 separate copies of the same library with every OS installation increases the attack surface, maintenance costs, wastes space etc... The store already has functionality for shared package dependencies but it hasn't been used in this case for some unknown reason.