For a velocity of 45 m/s and an arrow mass of 36 grams the kinetic energy is 36.45 joules. From the available information we can only estimate the energy that the archer expended in drawing the bow. If the distance travelled by the string (i.e. from the string's rest position to full draw, not the "draw length") was 0.43 m (i.e. 17"), the bow had a linear draw-force to draw distance relationship and 311 N draw-force then the input energy would be 66.9 J. This implies that the bow was operating at 55.5% efficiency, or thereabouts.
The original source does not say how their figure of 70% conversion of strain energy into kinetic energy was calculated.
55% is closely comparable with what I get for self yew longbows of around 70 lb draw-weight, and also with what I have calculated from published figures of arrow velocity, arrow mass, bow draw-force and draw length for 140 lb warbows. So, I would say that the Hadza bows are operating at about the efficiency that I would expect for a well-made wooden self bow, rather than the ~70% typical of modern recurves.
