Wing flex is an important consideration for most airplanes. Even the wing of a modern airliner will flex substantially, both in bending (tips up) and in torsion (tips twisted). Wing flex will also reduce aileron effectiveness at high speed; that is why Boeing used to add high-speed ailerons to their airliners.
Wing flex comes in two varieties: Bending means that the lift force will curve the wing tips up while the moment caused by the chordwise distance between the lift force and the elastic line of the wing will twist the wing. The high camber of glider airfoils causes large variations in the chordwise position of the center of lift, and since the effect of this torsion adds up over span, long aspect ratio wings are especially prone to torsion.
However, the largest torsion happens to occur at small angles of attack, going down as angle of attack is increased. Since the weight of the glider is constant, the same goes for the lift force, so torsion is inversely proportional to angle of attack. Stalling happens at high angle of attack when torsion is low, so it contributes only a little to the stall characteristics of gliders. However, in an accelerated stall or in a tight circle, when the wing has to produce more lift in proportion to the load factor, torsion can become significant and will unload the tips, reducing bending and increasing the stall margin on the outer wing.
Flaps have a larger effect on stall characteristics: For low speed flying they are lowered more on the inner wing, so it will contribute more to total lift and unload the tips. In the ASW-22 or the ASH-25 with flaps set to "L", the ailerons will actually move trailing edge up. If you fly those gliders at slightly more than approach speed, the wing tips will bend down from the negative lift on the outer wing.
See here for an answer demonstrating the effect of bending on stall characteristics.
Mr. Gudmundsson isn't directly wrong, but I think he exaggerates the effects of wing flex if he attributes the good stall characteristics of gliders to them. More important is careful airfoil selection and planform design: Glider designers know that their products will spend a lot of time near stall so they use airfoils with docile stall behavior and select taper and twist such that the tips will stall much later than the inner wing.