Imagine a wing that's 20+ times longer than it is deep, and is only 5% as thick is it is deep (so, for example, a 20m wide wing, that's 1m deep front-to-back, and 5cm thick top-to-bottom) - that's really hard to get stiff in bending along it's long axis, and in twisting stiffness around that long axis. This is why - as the article mentions, a sailplanes long thin wings account for 40+% of the airframe weight, when a stubby-winged but less aerodynamically efficient wing (like, say a Cessna 172) the wing might only account for 10-20%$ of the total airframe weight.

This TURN design minimises bending/twisting forces by replacing the load bearing and alignment forces with mostly end-to-end tension ones - which are much easier to resist (just load the structure up with "axial" carbon fibre...).