I have been a .NET developer for about 9 years, and I am a little dumbfounded by this article as I have integrated opensource whenever I could, whether from codeplex (or github) or codeproject. Of course, if you are developing an application, you are not going to incorporate one of the few libraries that suicidally licenses itself as GPL, forcing you to do the same with your entire application -- you are going to stick to LGPL and BSD type licenses. This is not .NET specific -- any real app developer doing commercial work is faced with this reality (unless they don't want to make money, or want to be exposed to lawsuits.) Another great option for library developers is a dual GPL/commercial license, to let users try a library and work with it internally before deciding to purchase.
Sure there has been a cultural shift that maybe was in more of a full swing from 2006-2010 that has seen open source as less suspicious and fringe and more useful, but my reaction is: 2009 called and they want their story back.
There is still a lot of irrational fear about .NET out there. I used to find it irritating but now I am amused by others' ignorance as .NET continues to do well and be an enjoyable evolving technology within the scopes of the markets carved out for it on the Windows desktop, Mono, and virtually every major gaming platform via Unity3D. (Unity3D itself is not open source but it was made possible by the open source projects Mono and MonoDevelop, and I was just looking at an opensource plugin for Unity3D today.) ...And this while Oracle, which doesn't even seem to hide their evilness, seems to let Java rot with security vulnerabilities and slower inclusion of language features, as I uninstall it and don't miss it or see a competing alternative to Unity3D. And I develop for iOS, where Apple's tyranny makes Microsoft look pretty soft. (And LGPL isn't compatible with Apple's app store thanks to the relinking requirement.) (Note: I develop for both iOS and soon Android using .NET.) Aside from server-side things and perhaps Android, as far as I can tell .NET is being a better Java than Java at the things I pay attention to, and all the open source plays a huge part in that. (I do wish WPF/Silverlight was better and more prevalent, but perhaps I will be inspired to make an improved cross-platform version myself someday -- but many who like WPF have liked it a lot.)
I cut the haters some slack though, as I used to hate Microsoft in 2000, and it took me a few years of full time work with .NET to get over it and realize I could enjoy the technology today and probably years to come, rather than bracing for some unlikely patent-mageddon scenario where Microsoft lawyers slice off one of Microsoft's own legs and eat it for dinner. Even if that day comes, I am prepared to adapt to another technology feeling I come out ahead. I also stopped being a cheapskate and realized money (non-free software) makes cool things happen, and paying $1000 bucks for something is often better than working 200 hours to get something lesser that I don't have time to maintain or start an OSS community around. (And some stuff would be out of the question for me to create, like Xamarin or Unity3D.) Yes, companies like Microsoft have done and will continue to do some stupid things, but if you take corporate stupidity and greed into account and take the time to understand the realm of likely outcomes, you can save yourself from drinking the FUD kool aid and looking silly. But then again, if you are happy (and making a living) in whatever technological bubble you are secluding yourself in, power to ya. I for one am happy in the bubbles of my .NET worlds, surrounded with OSS (my projects usually end up with way too many cool OSS DLLs in them), and making my dreams come true with it.