I heard once of a polish christian monk who's name a dont remember who was asked what God did before he created the universe. This was back in the 15th century or something when a standard answer would have been something along the lines of "He was preparing Hell for people who ask such questions." Instead he answered that time is a dimension of the universe, and as such God exists outside of time so the question has no meaning.
I have since thought that the same would be true of the enlightenemt ones. Since time is a dimension of the universe that we now call samsara, from which the Buddhas are liberated, a Buddha could traverse time as easily as we would walk down the road. We also know that it is possible to be close enough to enlightenment to achieve some level of clairvoyance and many ancient Tibetan practitioners did achieve such near enlightened abilities.
Building on this pre-supposition we can assume that to some ancient tibetans, time would be less constrictive than it is to most of us and so it is plausible that some of the most advanced practitioners would be able to see through time and prophesize the future.