It might have to do with the fact that he's using rubber bands to turn the arm, while historical trebuchets used weights. A weight is limited in how fast it can accelerate so you need to use a lever (the arm) to increase the speed.
Surely the above comment is confused, misremembering leaf spring as lead spring.
Leaf springs appeared as early as the Roman empire as a form of primitive suspension for chariots and other two-wheelers. Instead of steel, savvy Romans used flexible wooden sticks.
https://connect2local.com/l/112910/c/478618/examining-the-fa...
It raised some flags in my head when I read it, but then again - Romans did so many things out of lead that I did not challenge it.