Germany's Bild newspaper reports that Piëch told prosecutors he learned of the emissions cheating in February 2015, and brought it to the attention of then CEO Martin Winterkorn and a number of supervisory board members (Dieselgate came to light a few months later, in September). These other executives include Lower Saxony Prime Minister Stephan Weil, and Piëch's own cousin, Wolfgang Porsche. Board members "unequivocally and emphatically" reject the allegations.
Volkswagen is none too happy about the accusations, and is considering striking back. VW says in a statement, "The Board of Management will carefully weigh the possibility of measures and claims against Mr. Piëch."
If VW seems particularly rankled, it's probably because they've been down this road with Piëch before. In an investigation following similar comments from Piëch in early 2016, board members say his accusations were found by law firm Jones Day to be "implausible." And let's not forget there was that whole power struggle thing a few years back, which saw Piëch ousted from VW in favor of Winterkorn in April 2015. It's not hard to imagine why these renewed allegations would be met with some amount of bitterness.