The Importance of Sagan's Co-Authorship

Schlowski later wrote about how grateful he was to Sagan for being able to add things that he either didn't have access to the information because of just a great number of amount of censorship. So what could he express back then? Would it be something like the US has achieved certain technological achievements and have to pretend those haven't occurred? It's impossible to speculate on what someone did not include in the book. Historians are limited to what we actually have their historical record of.