I don't quite understand -- what is it about this technique that makes the trees grow perfectly straight and why is

> the lumber produced in this method is 140% as flexible as standard cedar and 200% as dense/strong,

?

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.

It’s exploiting the natural tendency of trees to create “waterspouts” through a technique called pollarding. When a tree suffers an injury it creates a bunch of new twigs that tend to grow straight upwards if the injury is on the upper branches. The waterspouts grow more slowly and so in this species of cedar they develop those desirable properties.