Somehow I come away thinking that twist-welding isn't true welding in any way.

Yes, the wire strands are being heated to a point where the material loses much of its resistance to deformation. Twisting, done at this point, imparts significantly more contact between the individual wire strands than what they'd had in their original layup as wire 'rope'. Once cooled the affected strands are held tighter together as a result; they ought to be separable still, if done with some care, but are likely to be more brittle.

Propane, burned in air, isn't going to achieve temps that will 'weld' stainless steel, but it will make it more malleable.

Tension, applied to pull the heated area apart, is simply taking advantage of the reduction in tensile strength from heating to separate one section from another.