I don't see his answer as any lack of faith in IT4. It is actually an excellent answer and says what we all know, that you need several guys who can score if you want to win titles, not just one guy. Red had said once you need 8 guys to win titles, Ainge is basically saying the same thing. We have a few guys, we need several more.
What if he had said a simple 'yes' and then he trades for Butler or Cousins in a few weeks? The team is then not built around IT4 anymore, as either of those two are equal or greater than IT4.
Saying a simple yes is also a slight at Horford, and possibly at guys like AB , Crowder and Smart, KO , Brown, and Rozier.