Apples and oranges. Look at what he's taling about, 'Truly I say to you, I have not found so great faith..' He's commenting on their faith in God - not where they got it from.. not the message that provoked such faith in God. That leads you right back to what you are trying to avoid, As Paul noted Romans 10:17 "So then faith [cometh] by hearing, and hearing by the word of God." Faith comes by hearing the Word of God. That would be scripture.
There are many other examples where He tells the people without quoting Scriputres what is really at stake and what they should know (and believe) by that same phrase. He is telling them that which He is thinking, not necessarily that which is written