Hemp ropes are quite strong, they've proven themselves for thousands of years in marine applications. You definitely need more thickness to achieve the same breaking strength as dyneema or steel cable but at slower speeds and higher projectile masses the aerodynamic consequences of a thicker sling rope might not matter so much. As for the "drive rope" for lack of a better term (the one that transfers power from the falling weight to the rotating drum) that one can be basically as thick as you like.

No, I understand, it's the claim from the very first sentence that "gravity itself seemed to forbid" (reiterated later in the text) that annoyed me. It's just making up a non-existent obstacle to hype up the result.

Of course, but there is a continuum between "I spent years tackling this complicated problem and the solution will benefit us all" and "I've entered the same McDonald's drive-through 1000 times in one day to order always the same thing". Some achievements are closer to the first end, others lie more somewhere in between.