In the update function of the ball the location is approximated by the secant (velocity * dt). This is not precise as the velocity changes. In complicating calculations such an approximation is necessary. In this case the exact value is well known even for beginners of physics. s(t)=att/2 . s(t+dt)-s(t) =a*(2tdt+dt*dt)/2. This and the other curve are a Parable. But in the case of the tutorial the acceleration has to be adapted.
Thank you for the link! They call “External Euler” what I call “secant approximation”.