No, because the premise is open source projects which developers, skilled professionals, work on.

My real hypothesis, by the way, is that designers have been trained from early on not to do free work. The “work for exposure” meme captures this pathos.

You do understand that designers do have free will and a developer can't simply order one to work for free on someone else's hobby just because he could use specialized hep, don't you?

It seems you're entirely oblivious to the very nature of FLOSS software, where practically all projects are born as a single individual's hobby, and some of them happen to grow way past the single-developer stage. With that in mind, what exactly leads you to believe that designers have no free will of their own and simply spend their days working on someone else's hobby just because he could use the specialized help?