Hello
Not sure what I did wrong, but when I followed along added a 'stretch-to' constraint to the DEF upper-arm, forearm, thigh, and shin, I didn't get the same result. The result I got was the same as when you just added the initial constraints to the forearm and shin, the whole arm/leg chain doesn't stretch.
Wow this was helpful! I had missed that too. Thanks for this question and the answer too :)