They had to build iTunes and Safari for Windows somehow.
Objective-C has bee around for a long time, and certainly isn't just for Cocoa/Apple.
Objective-C only continued to exist past the 80s because of Apple/NeXT. And the closest thing it has to a formal definition is the source code to Apple's GCC fork.
I like how whoever checked in the code included their .suo file, which is the user options for the solution!
If the way iTunes runs on Windows is any indication, I have no desire to develop for it on Windows. iTunes does run fine on my iMac, so I'm sure Objective-C is a fine framework... just not sure about the Windows implementation.
FWIW, iTunes is written with C++/Carbon, not Objective-C.
btw, I still have YellowBox (WebObjects 4.51) installed on my machine. Haven't used it in a while, but still....