Have you even researched the law on extortion since your post? Even superficially? From the threat of a lawsuit--even a frivilous one, is there a threat against the person? Is there a threat against his property? Can we have "extortion" without one of those?
(Actually we can, the statute in the OPs state is broad. However, even with its broad nature, the threat of a lawsuit, justified or no, probably wouldn't fit.)