Hard to prove, hard to disprove. Even without IP the sheer staggering output of such a large and at the time, effective corporation would not have been easy to copy in any reasonable manner. Enterprises certainly would not have trusted their support to people without the source code. Lack of IP, after all, wouldn't
mandate that Microsoft make their source available externally, and without that, the knockoffs aren't going to have a lot of luck doing anything but blind aping. Consumers might have had some copy-based knock-offs available, but frankly, they probably would have always had reliability problems and Microsoft would have made efforts to make those problems worse, many of which would have worked well enough to probably make the problem mostly go away.
I can't disprove that Microsoft depended entirely on IP, but I'd suggest that it's at least a reasonable theory that it really didn't.