One is subjective. Like when you're high up and you see the ground beneath you, you know it'll hurt if you fall down, so you're more careful moving around.
Then there's all the hidden dangers that kids don't think about or see[1], like a small gap where a cord from the clothing can get attached potentially leading to suffocation.
The point being, the first aspect is something the kids should be able to expose themselves to. It teaches them about risk, but also can give them a goal and a sense of achievement when they finally dare to do something and succeed. His point was that this is an important part of their development, which could have knock-on effects later.
However the playground should be designed such that any accident should not lead to permanent injury or death. Sure they might get a bit banged up if they misjudge, lesson hopefully learned, but nothing permanent.
He was often in Germany to study their playgrounds for inspiration, and this article illustrates nicely why.
[1]: https://www.utdanningsnytt.no/barnehage-grunnskole-leker/her...