If you believe Lumafield, 8% of low-quality lithium ion batteries have a mechanical defect that can sometimes lead to a short circuit.

Is this person really brave, or just unaware of the risks?

https://www.lumafield.com/article/finding-hidden-risks-in-th...

He put a fuse on every individual cell and on the overall unit, so I would say he was reasonably cautious (although he deployed a bunch of high-voltage exposed wires at the end of the video, but we can assume that was just a tech demo).