I finally received an answer back from Canvas support. "Reviews Due" does not mean the due date for peer reviews. It means the date at which peer reviews are assigned to students. If "Reviews Due" is left blank, peer reviews are assigned on the due date for the discussion. There is no way to assign a due date by which students should complete their peer reviews.

Canvas UI designers, please rename the "Reviews Due" field to describe the actual function of the field.

Hopefully this helps someone who has the same question.