Translation: they do not see the audience as a problem to be addressed, but an assumed set of factors. Now I will continue to talk about the problem for a few more pages and only respond with an answer at the very end.

This is a confusing construction. Summary: purely rational argument is trying to appeal to a "universal audience," but the editors want to clarify that Perelman and Olbrechts-Tyteca did not really believe that a true universal audience exists. Rather, a speaker imagines a universal audience (rather than one that already has a set of shared values that must be appealed to in a specific way), and then tries to make rational arguments that could appeal to the "universal audience" of their imagination.