Siona's visions were about the danger of presience and the need for mankind to become free from those who would use fate/predestination as a means to control mankind. Avoiding the predestination paradox was literally the reason Leto created her.

Given the lengths that Frank Herbert went to avoid having computers in Dune, it would not make sense for his big bad to be a computer revealed in the last few pages of the series, especially when his big bad was always intended to be Destiny.