There may be a threshold response, but from what I remember, the threshold might be low. I wonder if much of the fiber in tomato is destroyed when reducing it to a sauce?
Cooking doesn't destroy fiber, it just softens it. As opposed to pulp-free fruit juice, from which all fiber is removed, and is unhealthy. Even if tomato sauce were the equivalent of fruit juice, you'd usually have it with pasta, bread, etc. which all have fiber as well.