I'm perfectly familiar with Microsoft being an application vendor for Apple's platforms, long before that was Macintosh:) What threw me off with the notion of them employing a systems programmer for a non-MS OS.
It makes more sense when you consider that 'systems programmer' doesn't mean 'writes operating systems' (especially back then) and the line between application and systems programming was quite blurry (double especially back then).
Microsoft buying a big chunk of Apple (admittedly to stave off antitrust claims that there was no competition in the OS market) is a major reason Apple is alive today.
On the other hand, Apple unintentionally granting a perpetual license on their interpretation of the WIMP GUI (admittedly not without perusing the look-and-feel lawsuit) is what kept Microsoft in the OS business.
Also Applesoft basic was derived from Microsoft basic?
Yes, they did copy the operating system but that doesn't mean that the Mac Platform is unimportant to them.