One of the challenges we face while trying to answer the original question is defining terms like "code". I won't try to do it. Another challenge we face is trying to figure out when the massacre plan was formulated. It seems pretty clear from the novel that Vito started making plans right after the Dons meeting, maybe even before it. His first priority was to get Mike home safely, but then what? Massacre (although the novel limits it to two Dons)? Maybe. I think yes. The novel makes it clear that Vito was heavily involved in planning it. So, ask yourself, just what constitutes breaking one's word? Apple is convinved that Vito would have had to murder one of the Dons himself or to have directly ordered one of them murdered to have broken his word. Apple also seems to propose that the massacre wouldn't have taken place had the Barzinis not broken the peace first. I don't buy either. As Hagen observed: "All pointed to a day of reckoning in the future". Vito intended to murder Barzini and Tattaglia in good time and his planning of it breaks his word.

By the way, Tessio's betrayal was an aside. It added to the drama, to the pathos. That's all.