You are partially correct. According to the Glossary Performance Reports does include WPI but it also goes further than that because it includes Earned Value which is not part of the WPI. Also take a look at figure 11-20 where you can see that these 2 inputs come from 2 totally different processes.
So yes... there is definitely some overlap of the information in these 2 inputs and you could argue.
But remember... when it comes to the PMP Exam then the PMBOK Guide is always right. (I know that may not be a 100% satisfying statement, but it helps to pass the PMP exam.)