Wikipedia summarizes commodity fetishism
commodity fetishism perceives the benefit of a commodity to an agent as something that arises from commodities themselves, and not from the interpersonal relations that produce them
It seems to me that Marx is saying that
if Mary says that insulation is a benefit of (wearing) a sweater, then she fetishizes the sweater.
but if Mary says that insulation is a benefit of someone knitting the wool that somone sheared, then she does not fetishize the sweater.
However, if Mary is cold, all that matters is that the sweater insulates her. The causal chains that brought the sweater into existence seem irrelevant to her need.
(I suspect that that is part of what Marx's is driving at: if we remembered that people provide the benefits maybe we'd care more about workers.)
Marx's choice of the word 'fetish' suggests he takes issue with this kind of causally-blind thinking. Specifically, that we're doing something analogous to attributing supernatural powers to an icon.
Is Marx suggesting that by ignoring the causal chains that bring objects into existence, we fetishize their properties and dispositions, and thereby commit some error in reasoning?
Do I fetishize the metal lead by ascribing malleability to it? Would I not fetishize it if I ascribed malleability not to the commodity but to that which came to me by way of my encounter with the materials produced by stellar nucleosynthesis that the Earth gravitationally attracted?
What if I don't know the causal chain of an object? Is it enough to simply bear in mind that there is one? If so, is that not what we do naturally? (We all know that causal chains of labour or other events shaped materials into their current forms). If so, then what's Marx's beef with doing that?
(Answers written for an Analytic reader would be appreciated.)