In what sense do you not "own" your codebase if you release it under the GPL? 

There is no concept of property for information. By hiding your source codebase and the way it is being built (the same logic applies to your DAW music project, your video software editing file project etc...) you are making life harder for people who try to replicate features of your software, understand techniques behind it or pretend to be you to sell the software instead of you (others examples are welcome).

So I can understand that the GP /feels/ like he owns it more, even if it is just a feeling.