Fastjack was not, by his own admission, part of the Echo Mirage team, which included some of the best people and hackers he'd ever known. Galivan/Knight (obviously) was. While there is not enough proof for Knight to be conclusively and publically proven to be Galivan - by which we mean 'provable in a court of law' - there is enough evidence for the shadows to consider this absolutely true, and for those with connections to the shadows to be 'certain to the point of not needing further evidence'.