This is not a new idea. In Vedic philosophy this has been preached and practiced only by realized person. In fact, practicing telling only truth one becomes realized. For e.g Gandhi adopted this practice of speaking only the truth as that is one of the limb of Yoga.

What happens is the moment you take this resolve to speak only truth, your actions and thoughts become aware. So there is a very subtle refinement going internally as you keep practicing speaking the truth.

But I think it is more appropriate to speak only when you are asked about it. Just telling your thoughts just for the sake of speaking truth does not make sense.