Question about Split License for Wordpress Themes

In this article it says

If an author is releasing a theme or plugin for a platform that is licensed under the GPL, the PHP code and integrated HTML of the theme or plugin must be distributed under the GPL .So, when a buyer purchases an item from ThemeForest or CodeCanyon, and finds the GPL license text enclosed, this license applies to the item’s PHP code and integrated HTML. Other assets that are a part of the item may not be covered by the GPL

Does that mean that any custom code i do for my Wordpress Theme is free to use by anybody?
So if i work on a “custom feature that nobody has” and i release it in my theme any competitor can take it and add it in his own theme. ?