It means that he makes allusions to so many writers that most people have never read what they wrote.
Student or Learner
What's the meaning of the underlined sentence?
His was a complex, allusive style, often leaving more unsaid or hinted at than fully stated. It is not always easy to fill the gaps. One has seldom read every writer he alludes to.
Source: Terrance Brown, About the translation. In Jean Piaget(1985), The equilibration of cognitive structures: The central problem of intellectual development. The University of Chicago Press. p. xi.
A better title would have been One has seldom read every writer he alludes to.