There has been evidence to show that vitamin B1 is effective for reducing stress and helping in transplant shock in plants. Superthrive contains B1 so it should prove useful for treatment when transplanting etc or at other times when a plant is stressed. It is not a rooting hormone or fertilizer however.
Perhaps what people interpret as a boost in growth is related to superthrives ability to help a plant overcome stress and in doing so the plant grows better for a short time. If this is the case then a plant which is growing well in a stress free environment already and has a strong healthy root system will not benefit from periodic applications.