There are many kinds of developers. I am not a desktop developer, I am a web developer. I was a professional web dev for many years, now I am a hobbyist and code for my own needs.
So, while I know how to write for the web and how to write PHP, JavaScript, work with MySQL and stuff, I know very little about doing anything on the desktop (apart from coding some games, usually with the help of frameworks).
But you actually raised a very good point in that being a developer does not mean you are a developer of everything.
Therefore, open source desktop DSP program written in C++ is only relevant to a person who knows DSP coding on the desktop using C++. So, a given open source program is potentially interested only to a segment of developer community.