I think that your last sentence here is key. When you write that "Sallustius ... seems to be saying that Providence, in a material context, is called Fate for historical reasons," I think that's a nice paraphrase of the 4th paragraph of this chapter. Incidentally, I'd read this remark (yours, and Sallustius') as having an implicit swipe at the Stoics. Providence is the wider, more all-encompassing concept, whereas Fate applies only to the sublinary/material realm. But because the Stoics denied the existence of anything incorporeal/immaterial, they were unable to take the more expansive view from which Providence is "bigger" than Fate. Since for the Stoics, "material reality" is the only reality, their concept of Providence, to become a mere snyonym with Fate.