I disabled the link you added because they are not advertisers on PA. Folks can contact you for that info.
Last year(or the year before) at the Tennessee Classic I saw 2 different guys backing bows with raw flax fibers. It does make a good strong backing but it can overpower the belly because it has very little stretch. Trying to compare flax with sinew is like comparing apples to oranges. Sinew adds performance to a bow and flax adds protection like silk, linen brown bag paper or rawhide.