Hello Andreas,
I learned this the hardway, with a former co-author from another package of mine wanting to forcely take over the package by questioning absolutely everything in the package, from code to documentation.
After many iterations with the CRAN maintainers, I learned that the GPL licence from the DESCRIPTION file is only about the package as a whole and it mostly covers the documentation. As counter-intuitive as it seems, it looks like it does *not* cover the actual code inside the package. I was specifically made aware about this because I then wrote "I do have previous versions of the package before my co-author joined, and prove the code is mine to protect its IP" and they replied that unless I specifically mention a copyright statement (other than the package's in DESCRIPTION) the code is completely unprotected.
This is the story behind that statement in the R file, I chose one that is as open as possible (it basically allows everyone to use my code, with a simple retention of the copyright notice.
Now, about your specific situation. Do please make me understand, what is the exact problem under Debian? Do they not allow such copyright statements?
Best,
Adrian