The article seems to be taking the position that the C runtime library is not part of "Windows", which feels like a rather odd view to me. What is the stable API that Windows offers to application developers if not that?
Indeed, C runtime is not part of windows API, and it's normal to have a program include few different copies of C runtime library due to different modules compiled with different compilers/options.
C runtime library being part of OS is accidental thing in Unix, 16bit and 32bit Windows API even does not use C-compatible ABI (instead, Pascal-compatible one is present)
The Win32 API. E.g. using WriteFile to write files (https://learn.microsoft.com/en-us/windows/win32/api/fileapi/...)
It wasn't until fairly recently that the C runtime was stably shipped with Windows. Previously you had to install the correct version of the C library alongside your application.