The novel provides the answer here. In the novel, Puzo several times says that "no non-Sicilian could ever hope to equal a Sicilian in cunning." After Sonny was killed, Tom poured himself a drink and said to himself that he knew he wasn't a wartime consigliere (his exact words). "Genco would have smelled a rat." Why did he say this? Because, if he were Sicilian, he would have suspected that Carlo would never have lived with the humiliation Sonny inflicted on him without seeking revenge. He would have warned Sonny to look out for a plot exactly like the one that entrapped and killed him. Genco would have assumed that Carlo would have thirsted for revenge.