as we were watching, none of us thought that it was anything dan hardy really did right. we thought it was more that GSP was too nice. if he had reset and kept it tighter, i don't think the same thing would have happened.
I generally believe that when somebody can handle a really tight joint lock past average parameters, it's probably something very natural/genetic with their body, and i don't think people can train their body to handle the same submission stretches like other people. on the specific subject of the arm bar, it seems especially wrong because, assuming that all other joints were held constand, the elbow would be the only joint to take the pressure. i'm not sure how probable it is for most people to have naturally hyperextendable elbows, but it doesn't seem great.
when dan hardy was escaping, i believe he did the hitch-hiker escape, but i can't really remember right now. this would allow him to take use his shoulder flexibility to take pressure off the arm-bar, but theoretically speaking, if the armbar had been perfect, this escape would not have worked. therefore, shoulder flexibility would not have mattered.
it's kinda like saying "if we deadlift a lot, then we can escape triangles better." now if somebody does a bad triangle, then your deadlift strength could probably help a good deal in that situation. but if somebody does a good triangle, then you're probably not going to get the opportunity to straighten your spine out during the submission. therefore, your deadlift strength will go to shit.