You are 100% correct.
The misunderstanding that "linking" (mashing in the same address space) a derivative work does is widespread. But it is a misunderstanding anyway.
People who pay attention to my comments -- if they exist -- know that I am very pro-FSF but that specific piece "linking a derivative works does" of the GPL is bollocks. I have analysed it thoroughly.
Now, if _the_ Pierce themes are derivative works of WP, that is one thing that I didn't analyse myself. They can be (especially if lots of non-trivial, can-be-done-differently-but-wasn't boilerplate code is involved). But I have a serious problem with the linking myth.