he alternates good years and bad decades

I think you reach the same conclusion coming from either direction. On one side, you see Chen having moderate success despite lack of “stuff”, and he even claims that using different arm angles and throwing junk helps him squeak by – so you wouldn’t be surprised to learn that his BABIP has consistently been better than average. On the other side, you see that while his FIP stats have been not good, his BABIP has been good – so you would expect that he uses some non-traditional pitching approaches.