If that API is under the GPL then isn't use of it a derivative work? If the API is a GPL implementation of a standard API, e.g. the JDBC API, then it wouldn't be a derivative work, not sure if Themes call a wordpress specific API or not though.
I think that's a great question, and one that could probably spawn a ton of interesting legal/logical arguments.
At what point does dynamic linking make something a derived work - and how is dynamic linking fundamentally different, than, say, a remote procedure call (or SOAP or REST or whatever) over a network? The interface is different, but the principle is the same.
Also - the GPL does state that it's okay to link against things that are "standard parts" of the target system....
