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I believe the big thing is that while the Cabal's Prescience is powerful, its not perfect, and even with them convincing the Alpha Legion to side with Horus it was not enough to prevent the Emperor from being victorious. I may be wrong about this, but from what I've picked up the Tyranids were outside of the calculations of the Cabal, possibly because of the shadow in the warp effect, which may have twisted the accuracy of the prophecy. As for their current actions, don't forget that their goal was for Chaos to wipe out humanity, which will in turn deny the Gods their primary source of 'food', starving them out, so continuing to side with Chaos could be argued as an attempt to still bring about the ultimate end of the prophecy. Alternatively, giving the corrupting nature of chaos, it IS possible that over time all or at least parts of the legion (which is scattered now to a degree) have succumbed to the influence of chaos, no longer seeking to undermine it from within but instead acting as loyal servants.
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