I don't see any reason to doubt that Krell had an accurate vision of the future, which would only need to mean that he saw a sliver.
But what reason is there to believe it? As I understand it, precognition, while not unprecedented in powerful Jedi (cf. Anakin, Ahsoka), is a rare ability. It seems more probable to me that it was just a figure of speech expressing his belief or agenda, the way it is 99.999... percent of the time you hear people (especially power-mad fanatics) intone "I have seen the future!"