Dyneema has remarkably low elongation, in addition to its very high strength to weigh ratio. Higher than Kevlar (but less heat resistant). It’s why they used it for sail cloth before rigid body sails took over most competition sailing.

Aaed I believe is preloading his devices, which should eliminate most of the remaining give but also increase wear rate. Though it sounds like from the video that the biggest problem is rubbing the cord against itself, hence the channels to guide the cords.