In theory, this developer could just use Mono to run his .NET app on Linux. But that requires them to learn how to do that. Personally, I've never found a clear tutorial on how to take a .NET GUI app that you've just written in Visual Studio and get it running on Mono. I haven't looked in several years, so I thought I'd give it a try right now. I googled "visual studio mono" and these were the top 3 results:
http://www.mono-project.com/Visual_Studio_Integration
http://www.mono-project.com/Working_with_Mono_and_Visual_Stu...
http://www.mono-project.com/GettingStartedWithMonoTools
None of those are clear at all. The second page links to this screencast: http://www.mfconsulting.com/product/prj2make-sharp/tutorial/...
which illustrates how to use this thing with Visual Studio 2003(!) and demonstrates part of the problem: the Mono version of the app looks noticeably different from the native Windows version. Sure, this can probably be fixed, but it's yet another issue that the dev has to spend time learning how to hack around.
This situation results in substantial cognitive overhead. It's unlikely that an average .NET dev will be able to go into work, install this Mono plugin, compile their app using it, and present to their coworkers, "Look! We don't have to change our process at all, and we can deploy our app on the latest Ubuntu with a minimum of hassle."
It's hard to imagine how wonderful it is to build .NET apps compared to dealing with Qt, because Visual Studio's GUI designer is just so good. Suffice to say, there are a lot of network effects that keep a developer mentally locked into Visual Studio's paradigm of "here's how you make this GUI form;" a paradigm which is immensely difficult for Linux to support natively.
I think it's a mistake for Linux to even try to support this, though, because it's probably focusing on the wrong problem. The way to beat Microsoft is to make it so easy to write cross platform business apps that newer generations never even bother figuring out how to install Visual Studio. I think the web browser will be that solution within the next decade. It's not quite there yet -- it's way easier to get a GUI workflow up and running using VS's designer -- but it seems inevitable. So if you share the author's concerns, then the best course of action is to write some quality tutorials for your web frameworks, and to invent easier frameworks. The advantages of the web will naturally outweigh any possible native program advantage.