Water and a 50/50 mix do have boiling points. In some areas of a vehicle, that point can be reached, and the coolant boils. Of course, that heat is in the form of air, and it doesn't do a very good job of transmitting the heat from air to water vs from the engine to water.
The size of the bubbles can be reduced or eliminated by raising the boiling point or by reducing the surface tension of the coolant. If it doesn't boil, the heat continues to go into the liquid, and it carried away. 50/50 mix will increase the boiling point, but it doesn't shed the heat as well as water. Some propylene glycol based coolants can be ran straight in some vehicles making the boiling point over 300F. It doesn't shed the heat out of the coolant as well as water, again, but it pulls the heat out of the engine. We've used coolants like that in racing, where they are legal by the rules, and we've had very high coolant temperatures on hot days while the block remains a lot cooler than what we'd expect given our abuse and the outside temperature.
Surface tension is the method for stuff like Redline Water Wetter. Was originally used to reduce corrosion in fire truck water pumping units, then they found that it also would increase a coolant's ability to pick up heat, even water, by reducing surface tension further.
Fire units found that using an equal amount of another surfactant would do the same thing. Similarly, similar results were found by some race teams, and I've done it quite a bit. Basically, we've used Palmolive dishwashing soap in the coolant to increase water's ability to pick up heat.
The additive might just be some kind of surfactant. What's it called?