Fundamentally yes, though it hasn't really been tested in court and I'm not sure that plugins really are obviously derivative works - you can create a plugin th

Fundamentally yes, though it hasn't really been tested in court and I'm not sure that plugins really are obviously derivative works - you can create a plugin that doesn't depend on WP but can be used with WP, but I'm not a lawyer. Matt and WP's position however includes that this applies to the PHP code but not to accompanying assets like images, CSS, JS (because those can obviously be used without WP). Those only need to be GPL if you want to host it on wordpress.org which the commercial versions of those freemium-plugins are not.