The water bath in Kiinde Kozii is thermostatically controlled to the temperature of warm tap water (60C/140F). Due to manufacturing tolerances, etc, there may be a few seconds that the water bath temperature does get slightly higher than 60C, but due to the thermal gradient across the container the milk is in (bottle or bag), you can safely assume none of the milk is exposed to that temperature.
When warming with Kiinde Kozii, while the water bath may be at 60C, the inner wall of the bottle is many degrees cooler. All the milk in the bottle or bag is at roughly the same temperature (i.e. no hot spots), due to the gradual, uniform heating, the low temperature of the water bath, and the high thermal conductivity of the milk itself. As long as the user ensures that the warming time is not set excessively high, the bulk temperature of the milk will not exceed 40C.
Steam warmers apply 100C to the outside of the container. Several degrees of temperature drop across the wall of the bottle will leave the inside wall of the bottle (the part that contacts the milk) quite hot. Even with high thermal conductance, there will be a much larger volume of milk that is over 40C.