It’s not. Come from a whole family of sprinters. The start and the ensuing 10 - 15 meters has the biggest impact on your time, especially for people who are already pretty dang fast.
This 40 time talk is just dumb. The biggest factor affecting these kid’s times are their starting technique rather than their actual speed. A 4.8 kid could drop 2 tenths off their time simply by improving their starting technique.