That’s an approximate rule of thumb. The definition of a newton is the force required to accelerate 1 kg by 1 m/s². The ocean level gravity of earth just happens to be around 9.82 ≈ 10 m/s², thus a 100g weight feels approx 1N of gravitational force.

I’m pretty sure it hasn’t been 3st of August yet.

https://archive.is/VjQuZ has the text. Even better, the beginning, which I presume to be one of these terrible scroll-to-advance animated presentations, has the animation removed.