No, that's not founded on reality. Windows has utilized existing open source code in it's evolution, btw. BSD networking, for example. Windows would never have existed if IBM hadn't published the BIOS source code (long before Windows), IMHO a stroke of competitive genius. The fabled competitive power of proprietary closed source is a myth long exploded. It only works for a few things, for a limited time. The required pace of innovation to fuel the continued sales of a software brand can not exist without a open marketplace of ideas, a marketplace which exists in the minds of people. Ultimately the premise of "intellectual property" asks us to control what people may think...how well do you think that turns out?
So Windows was never about the source code, it was about creating value in the mind of the consumer and a marketplace for the exchange of value. That was the product Microsoft created. There are enough people not served by the Microsoft paradigm that the open source approach has found a steady stream of adherents. Microsoft and other commercial software vendors have repeatedly have repeatedly attempted to poison the well in hopes of artificially inflating the value of their products but the reality is the sophistication and value of open source has managed to overcome all efforts to stuff the genie back into the bottle.
It worked long enough for Gates to make enough of a profit.