His argument is solid. What other PC operating system supports as many hardware devices, has as much software developed for it? Regardless of the OS being open, Windows as a platform has been a very open to anyone who wants to develop for it.
Most open though? More open than Linux? Seriously? Not a chance.
Vendors (hardware, software) supporting Windows is not the same as Windows supporting the vendors. Linux supports more hardware than Windows out of the box. Linux supports all those non-out of the box pieces of hardware just fine, the vendor just has to write the drivers for them.
Windows is proprietary to Microsoft and doesn't implement an open standards backed API. Linux supports proper open standards, such as POSIX, and provides all the source so it can be extended.
By any definition of open, Windows does not meet it.