Are you two talking about a different Don Vito Corleone?

After the Don swears that he will not be the one to break the peace he says "This is my word, this is my honor. There are those of you here who know I have never betrayed either".

The Don was a man that defined himself by his code and the honor of his word. Those words would not have been said by the Don, particularly in that forum, if they were not true.