Welcome. Anyway, no need to say thanks in a Q&A forum. That's what accepting answers is there for :)
We don't really know if there had been any kind of agreement between Gideon/Scott and Jatin regarding getting the theme into the core. I think there was none. Anyway, the theme is GPLv3 so as long as the core stays GPL too and the copyright marks from the comments are kept it should be fine. I believe it is also possible to remove the copyright marks from the HTML itself and distribute it that way too as you are allowed to change the code. Anyway, I find no reason to do so.
When I fork a GPL project I leave the copyright in comments as the author made them but I updated the metadata this way "Author: John (extended by pupi1985)". If I consider I made significant changes (e.g.: rewritten 50% of the logic) to the code then I also add myself as another author to the copyright in comments. That's the most you can do. Nobody can replace it's name in the copyright comments but it's possible to add more. A good related practice is adding a changelog that details the changes from your version and the author's version but as forking a project already links it to the original one I don't do so.