Adam,
Historical materialism does view historical production as linear
progression. Historical reality is seen as qualitative differentiated,
full of disjuncture, novel emergence, indeterminate outcome. Marx sets out
heuristic models with history only in broad lines and then suggests we do
grounded concrete analysis for empirical instances. Marx himself claimed
there were no overarching transhistorical laws of historical development.
I believe what is being presented here is a mythology about the
materialist conception of history.
Andy