Of course the code is theirs - You cannot take code from any other website weather it is a theme here, another marketplace or even Apple - change the design and reskin it. It is copyright theft.
In fact it is even in the other marketplace’s terms and conditions that what you are doing is not allowed.
All files submitted here need to be your own code, your own design (that doe snot include moving hsortcodes around), and original.
Aside from the illegality - these themes are not up to the standard for submission here, they don’t comply to Themeforest’s submission guidelines and if it is not code you wrote then how are you going to be able to update or support files in the future?
Unless you can create your own work then you will never be able to submit files here.