As the density of the air falls with altitude, an airplane's stalling speed (true, not indicated) rises. Conversely, as the temperature falls, so does the speed of sound. The convergence of these two values puts a practical upper limit on the altitude of a subsonic airplane. The glider's edge here is in having a low stalling speed.

https://en.wikipedia.org/wiki/Coffin_corner_(aerodynamics)

That's probably why their website says "PHASE 3 This phase will set a goal of exploring the stratosphere up to 100,000 feet. Flight speeds will increase to the point where the glider will need new transonic wings. Flight operations will be extended to exploring the Polar Vortex in the northern hemisphere."