My take on it, for what it's worth:

Work hardening requires overstressing a metal. Low carbon steel and stainless steel are two that can work harden rapidly when deformed beyond their normal stress tolerances. Stropping can't possibly provide enough heat and stress to cause work hardeining.

Instead, consider this. Stropping DOES remove and shape th metal. It does it at a slower rate than hone, by far, but leather will definitly polish metal and polishing is caused by removing metal at a micro-scale. This forms a very fine micro bevel, which is more obtuse and, therefore, more risistant to dulling. Until the micro bevel becomes so large and obtuse as to restrict the cutting ability of the edge, the edge will continue to become a little stronger and a little more durable with each time the blade is stropped.