Those are falsehoods told to corupt and damn the souls of those weak in faith. The Alpha Legion was fooled by chaos-tainted xenos scum and their human lackeys, the Soul Drinkers only delude themselves that they are loyal, but their coruption is clearly evident by their unholy mutations.

Tolerance breeds treachery. The Alpha Legion tolerated the false portents of xenos filth, the Sould Drinkers tolerated their mutations, both fell into damnation of treason.