Earlier quoted context omitted.
"The license explicitly says it has to be GPL. End of discussion. He is making up his own rules!" The license does. However, I don't think his theme uses actual wordpress code beyond function calls. I would hardly call this a derivative work.
It does actually go deeper than that, but really that's beside the point. Function calls into the WordPress code is enough. WordPress has a public API... several of them, in fact. A theme doesn't use those, it makes direct calls to internal WP code, and it uses WordPress internal data structures. This is enough to make it a derivative work. From http://www.gnu.org/licenses/old-licenses/gpl-2.0-faq.html#Me... : "Combi…
There have been no court cases I'm aware of that have dealt with this in the context of a GPL program. There are, however, numerous cases where the code being called was not GPL, and they have pretty much universally ruled that the FSF position is wrong--blob of code X calling into blob of code Y does NOT make X a derivative work of Y.