Ridiculous nitpicking. The difference between 'A extends B' and, for example, calling 'B::x' from inside a method of A is rather trivial, so does that also make you subject to the GPL? If so, what about calling B::x through eval()ing a dynamically generated string? People can argue semantics like this for weeks, there's no end to it.

Are people really this desperate to force their license of choice on others? No harm is being done to Wordpress by this, and if they really care, they can switch to a license that explicitly states how they wish their product to be used.