A derivative work pertaining to copyright law, is an expressive creation that includes major, copyright-protected elements of an original, previously created first work.
So if the Template includes code from wordpress then it is a derivative work, If the Template does not have any code from wordpress then it is not a derivative.
This has been well tested in the street mod automotive industry. If the new fancy street mods are not derivatives then neither is this.
I guess this guy would say every article and image posted on a wordpress site are now GPL as well.