Thank you for doing their job right. To be fair, desktopmetal.com is at least being linked below, but after clicking it I simply felt mislead, consequentially closing it again.
Fibrous composite materials can be significantly stronger than the source materials themselves. The binding agent forms a matrix which holds fibers together and so can be strong in localized fashion to hold fibers. This strength would usually result in the binding agent becoming brittle as a bulk material.
The overall strength comes from the combination of the two, with increased fracturing/tearing resistance due multiple independent fibers. Tears have a harder time propagating when each fiber needs to be fractured. Materials "fail" by cracks or fractures forming. In homogeneous materials the energy to continue a crack is generally much lower than continuing a tear. Sort of like static vs dynamic friction. Once started a crack will continue with a lot less effort leading to lower material strength. Fibers with a binding agent help prevent this effect.
A fantastic example of this effect is Pykrete which is just sawdust mixed into water before freezing. It becomes stronger than concrete (in tensile strength): https://en.wikipedia.org/wiki/Pykrete
I don't think saw dust is considered fibrous even if a tree is fibrous... I would think that to be considered fibrous, it would have to be longer in one dimension.
Saw dust isn't an ideal fibrous material, but it's still primarily composed of fibers [see 1]. Really they're a bundle of fibers. On a macro scale it makes less sense, but on a micro-scale the fibers are much longer than the micro-cracks that begin to form in materials under strain and which lead to the loss of strength or fracture we normally think about.
The technology is Forust: https://www.forust.com/
(why are neither linked in the Gizmodo article?!)