One of the underlying themes of the movie was that the built up legends of the gun fights as told in Sal Rubinek's books were really just seedy bushwhackings. Then, during the majority of the movie, Little Bill won every fight by getting the drop on someone and beating the shit out of them while someone else held a gun on them or they were tied up. The two people that Eastwood and the kid killed were caught (in one case, literally) with their pants down.
Then, at the end, William Muny gets drunk, rides into town, goes into a bar totally outgunned, and utters the line, "I've killed women and children. I've killed just about everything that walks or crawls at one time or another. And I'm here to kill you, Little Bill, for what you done to Ned."
After everything was said and done, at the end of the movie, was the one real gunfight for him to write about. And it was totally true.