This article is just a rehash or summary. Check out one of the sources it links to (since the other is broken) for details on the technique: https://mymodernmet.com/kitayama-cedar-daisugi/

The strength & flexibility I would guess are attributable to the lack of knots and the straightness of the grain.

One thing both writers keep doing that's annoying is calling it a cedar. The tree is cryptomeria japonica, known as sugi, which in English is sometimes known by various misnomers such as "Japanese cedar" and "Japanese redwood," both of which should be taken as more poetic than scientific.