By the power of truth, I, while living, have conquered the universe.
I've seen a lot of people explain away some of what the Timeless One says by saying "he's lying".
Having the Timeless One lie about these massive plot points which partly form the basis of 3 entire novels would just be a foolish and pointless move. While I agree we should certainly take some of his words with a grain of salt, I find it far more likely that he's using the truth as a manipulative device rather than lies.
Going back to the connection I examined between Gravemind and the Timeless One, we can all surely recall that Gravemind never lied - every word he said to John, Thel and Cortana was the truth, and he used that as a manipulative tool which even had us end up working with the Flood temporarily before being betrayed.
I'd say the TO is using these same tactics. I recall it being mentioned in Cryptum that the TO made a group of Ancient Humans commit suicide before its very eyes because he told them something absolutely horrifying, implying he said what would happen when they're infected by the Flood. The truth hurts, especially when it unmasks a lie - for example, the Flood cure not actually existing and the Great Journey being revealed to Thel by the Gravemind as a lie. The truth is a far more effective weapon utilised by the Gravemind/Timeless One, so I don't think his words should be so quickly dismissed as being lies just because the reader can't make sense of something.
[Edited on 07.12.2012 10:45 AM PDT]