While I have never attempted it, nor do I recommend it, a brewer could get away without boiling a starter if he/she pitched a large healthy culture. The need for sterility does increase as the number of viable cells diminishes due to the fact that the bacteria cells count increases eight-fold (8n) every time the yeast cell count doubles (2n).
Let's look at the definitions for the terms sanitary and sterile. Sanitary means that most of the vegetative cells have been killed. On the other hand, sterile means that all of the vegetative cells and all of the spores have been killed. Boiled wort is between sanitary and sterile in that boiling kills all of the vegetative cells, but does not kill spores.
A liquid has to be raised to a temperature of 121C/250F and held there for 15 minutes in order to denature spores. That temperature is impossible to reach at sea level because water changes phase from a liquid to a gas at 100C/212F at sea level (lower temperatures at elevations above sea level). The phase change from a liquid to a gas requires a specific amount of heat potential. This heat potential is known as the latent heat of vaporization. The latent heat of vaporization of liquid is bounded by pressure. If we raise the pressure under which a liquid is subjected, we increase the temperature at which the phase change to a gas occurs. Autoclaving works by raising the internal temperature of a vessel above normal atmospheric pressure. The phase change from a liquid to a gas is delayed until water reaches 121C/250F when the internal pressure of an autoclave or pressure cooker reaches 15 pounds per square inch (psi) above sea level atmospheric pressure, which is 14.7 psi, making standard autoclaving pressure 29.7 psi or a little over 2 bar.
As an aside, the temperature at which water changes phases from a liquid to a gas decreases one degree Fahrenheit for every 540 feet above sea level; therefore, brewers who do not live on the coasts should not expect their thermometers to read 212F when water boils. Water boils at 211F in my brewery.