It's kind of a subtle argument that I'm paraphrasing. Postone thinks (and I agree) that with modern antisemitism the Jew represents figuratively those aspects of capitalism that fascism wishes to denounce. Crucially, not all aspects of capitalism are being criticized! It's essential to this argument that national socialism criticizes only capital's mediating features, the way it illicitly crosses national boundaries, the way it magically multiplies itself, etc. What is not criticized are sturdy and dependable things like industrial capital, factories that produce heavy machines and weaponry.
Here is Postone explaining it pretty clearly in the essay I linked:
"What characterizes the power imputed to, the Jews in modern anti-Semitism is that it is mysteriously intangible, abstract, and universal. It is considered to be a form of power that does not manifest itself directly, but must find another mode of expression. It seeks a concrete carrier, whether political, social, or cultural, through which it can work. Because the power of the Jews, as conceived by the modern anti-Semitic imagination, is not bound concretely, is not “rooted,” it is presumed to be of staggering immensity and extremely difficult to check. It is considered to stand behind phenomena, but not to be identical with them. Its source is therefore deemed hidden—conspiratorial. The Jews represent an immensely powerful, intangible, international conspiracy. A graphic example of this vision is provided by a Nazi poster depicting Germany—represented as a strong, honest worker—threatened in the West by a fat, plutocratic John Bull and in the East by a brutal, barbaric Bolshevic Commissar. Yet, these two hostile forces are mere puppets. Peering over the edge of the globe, with the puppet strings firmly in his hands, is the Jew. Such a vision was by no means a monopoly of the Nazis. It is characteristic of modern anti-Semitism that the Jews are considered to be the force behind those “apparent” opposites: plutocratic capitalism and socialism. “International Jewry” is, moreover, perceived to be centered in the “asphalt jungles” of the newly emergent urban megalopoli, to be behind “vulgar, materialist, modern culture” and, in general, all forces contributing to the decline of traditional social groupings, values, and institutions. The Jews represent a foreign, dangerous, destructive force undermining the social “health” of the nation."
"When one examines the specific characteristics of the power attributed to the Jews by modern anti-Semitism—abstractness, intangibility, universality, mobility—it is striking that they are all characteristics of the value dimension of the social forms analyzed by Marx. Moreover, this dimension, like the supposed power of the Jews, does not appear as such, but always in the form of a material carrier, the commodity."
I should also clarify, he's not arguing that this misdiagnosis of capitalism is the origin of anti-semitism as such. He's arguing that anti-semitism (which obviously has a very long history in Europe) took on a new form in modern capitalism.