Another beautifully put paragraph.
Jameson is dealing with the objections, that we encounter too, about creating "periods" (aka stages or phases or paradigms in our onto-social contexts). Because, of course, in any study of cultural phenomena and cultural evolution there are no completely sharp breaks. It is always, at least when zoomed in, evolution rather than revolution.
But that misses the point. Good theoretical (e.g. dynamical systems theory) and empirical reasons suggest that we do have more discrete changes.
His second point is that this genealogical not linear. - it is branching and refolding Again of course, though a simplified genealogy is a linear one (e.g. the kings/queens of England).