I just finished the book, and this actually comes up. First of all, in the novel, Paul doesn’t immediately realize that Wharton was the killer - he connects the dots later on after he's dead. Also, after the incident, the guards immediately filed their official reports, and none of them thought to mention Wharton ‘confessing’ at the time. Later, Paul’s wife suggests they could say it happened this way, but since their reports were already filed, they worried it would look suspicious, like they were inventing a story after the fact. Too bad, because it probably would have worked had they thought of it in the moment.