This post sems to me to raise interesting points. I would rephrase
the writer's point: Capital is a theory of commodities AS products of
capital--Marx says this quite explicitly. Likewise, the theory of
value, as Marx also says, applies specifically to capitalism as a
system. It is only in this mode of production that abstract labor as
a social category comes into existence, since this is a
(mis)representation of the social reality of waged labor. Once the
category exists, of course (Marx points out in the introduction to
the Grundrisse), it can be applied in the analysis of earlier
systems, or even in imagining a future. But it could not develop in
earlier societies, so that any analysis of (say) the Roman economy
might use the concept of "labor" but would have to incorporate the
historically specific categories by which practices of production and
distribution were then structured. So that the differences between
capitalist and precapitalist commodities (or money) would be as
significant as the similarities.
Paul Mattick