Yes, you got it right. The source code is published under the GPL, but App Store ToS impose additional restrictions that are incompatible with the GPL. So we have to dual-license the project, and only the copyright owner can do that. In order to maintain that role, we can only accept contributions with a CLA (two pages of legalese that transfer the copyright). This is obviously a deterrent for contributions: over the 5 years, I believe there were only 3 people who signed it :)