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.
And they can be quite formidable, a full size wooden (yew, maple, ash) bow takes a lot of arm strength to properly pull. Just a few shots and you'll be wishing for stronger muscles.
Lead springs? They'd have to be absurdly oversized by modern standards to hold up to any kind of force without deforming, I'd imagine. I'd love to read more about them, if you happen to have a cite handy.
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.
