The secret is the overlap, which spreads the force.
In three trials, the webbing failed every time.
100%, 108%, 103% of manufacture breaking strength. Granted, this batch was probably above the stated minimum. Still, I consider this to be a perfectly reliable join. Another investigator (current) working with sewing machines uncovered the idea and got the same result; nearly 100% efficiency joins.
There are about 24 full stitches of doubled 40-pound test whipping twine (96 strands), giving a 62% stitching efficiency; probably more than were needed, and I would add even more to a project to insure durability. But you get the idea.
Do NOT use high strength thread (Dyneema, Spectra, Kevlar)--they will only weaken the project, lacking sufficient give.