Firstly, whatever Bishop Williamson's views were re: the Holocaust, he was a validly albeit illicitly ordained bishop when he made them public. His views only became public long after his ordination and there is no precedent for the absolute revocation of Holy Orders, a bishop is Sacerdos in æternum.
If I wish to deny the holocaust then am I not entitled to my belief?  Austria and Germany have sinister legislation which can gaol you for what you think. If I believe the world is flat or that the sun revolves around the earth is that sinful?
Four bishops were consecrated validly but illicitly and therefore incurred excommunication because their ordinations were not sanctioned by the Pope. Benedict decided to lift the excommunications and even had he known that Williamson denied the holocaust he could not have left him out as he was not excommunicated on account of his views on the holocaust.