I am not sure I agree with this analysis. Stallman convinced someone to open-source his entire lisp compiler because the REPL used the GPL'd readline library, and Stallman seemed like he would have taken the issue to court. Wordpress themes depend on Wordpress more than a compiler depends on a line-editing library, so theme authors might want to assume that they only have the protections of the GPL on their side. Anything else may be accidental.