Is a division of labor unacceptable here? Quite a few books entirely devoted to examinations of postmodernism have been written by Marxists, such as Ellen Wood, Alex Callinicos, Terry Eagleton, Norman Geras, among others, besides many articles and chapters dedicated to the same task. It is not as if Samir Amin, as an individual Marxist thinker, has to do the same all over again, unless he has something new to add to the existing criticisms.
On the other hand, I haven't read any postmodernist discuss articles and books by Wood, Callinicos, Geras, etc. Perhaps postmodernists consider them irrelevant or beneath their attention.