alt Hacker NewsIt's a bit clumsily written, but I'm pretty sure that's a reference to what Tom mentions in his video, that since g = 9.81m/s^2 you're not getting close to the speed of sound just from falling from a reasonable height, even ignoring air resistance.
Thus a traditional trebuchet design cannot reach that speed.
But since potential energy from gravity is mgh and kinetic energy of a projectile is 1/2 mv^2, calculating height and mass of a weight to power the projectile is absolutely straightforward. The terminal velocity of a falling weight has nothing to do with it (even historical trebuchets used a leveraged system in any case).