So out of personal experience and anecdotal evidence, I would say that the conditional probability of a voluntary Windows user being a great coder is less than, say, a Linux user.
If this is true, it's only moderately true and only recently. Historically Windows had a pretty big advantage in terms of great coders. Including virtually every game shop almost exclusively (incuding legends like Carmack, Sweeney, etc...).
It was in part just due to the fact that if you wanted to make money, you wrote Windows apps. It's like iOS is today, except a magnitude more dominant.
It's only in recent years with the web and mobile where this has begun to change. But I'd say that only mobile has typically had super strong devs. The web until just the past year or so has not been a place you could hire generalists. We've really just begun to see devs that I'd hire to do any work at our company -- not just web people.