In a hypothetical situation, someone could rewrite Wordpress under a non-GPL license, and make it support all themes. Irrespective of what Wordpress authors would say about the Wordpress clone itself, any themes used with such a Wordpress clone wouldn't be required to be GPL.
The Wordpress author's dangerously attempting to take away users' freedom to load code from different licenses and run it. Copyright and the GPL only limit distribution of the derived work, not the creation and personal use of the derived work. You're allowed to do anything you want with GPLed code as long as you don't distribute it, since keeping code for yourself isn't covered by copyright law.
"Conversion, fastidious Goddess, loves blood better than brick, and feasts most subtly on the human will." -- Virginia Woolf, "Mrs. Dalloway"