I see them benefiting immensely. A big problem that Microsoft has is that once you start going down rabbit holes on Windows the number of people out there who have a clue is vanishingly small. So for systems programming on Windows there is very little coverage online. Opening Windows up will broaden the number of people who understand the platform on a deep level.
Earlier in my life I did a lot of Windows system-level programming, including some minor driver development, and now I'm practically exclusively on the #nix bunch, sometimes doing similar stuff.
I can't say that there is a disparity. If anything, the parts that are documented are usually better documented in the Windows world, and there are very good books about most components. (To be fair I never read a book on anything unix apart from Lion's). There is a large amount of stuff online for either, but I often have the feeling that correct examples are less frequently found for, esp., Linux.
I've never done kernel work on Windows but on Linux at least you can dig into the source code to figure out how stuff works when the documentation is lacking. I suppose that's what the parent was talking about.
Not giving the source code forces you to write a better documentation though. In an ideal world, both would be available but personally I prefer a good documentation rather than source code diving.
Windows System Internals are probably the best documented in the world, MSDN alone is a treasure trove of information and there are countless books on the matter.
And unlike some other platforms what is written tends to match reality, and stay relevant for a long time (for better or for worse).
Sun is a beautiful case study in what happens when you're a little too aggressive about the "openness" mantra. They bet the farm on SPARC and lost. If they had kept pieces of Solaris closer to chest, who knows what would've happened, but they would've at least had one more potentially-viable commercial offering.
I assume that Microsoft is not going to be naive enough to do the same.
IMO Microsoft's interest in openness revolves around remaining a viable option in the cloud era. They need to protect their investment in the .NET ecosystem and MS-centric development workflow, which means making it easy for people to run .NET programs on *nix-based VPSes and containers.
AIUI, Sun bet on being able to sell stuff (not just SPARC) to FinTech firms. They borrowed money at the height of the bubble, and when the bubble burst, their customer base cut back on hardware purchases. Because they couldn't service that expensive debt, they had to find a buyer.