It was a very sloppy arm bar. When I saw GSP compete in Abu Dhabi in 2005 he was easily submitted in the first bracket. He would have never qualified for the Abu Dhabi if he had to earn his way like most everybody else. Being a top UFC contender he got a pass due to crowd appeal and giving the show some gravitas. Standing next to him in person I was surprised at how big he was. But it just goes to show that Jiu-Jitsu and ground grappling, though necessary, is insufficient when it comes to MMA and fighting when punches are thrown. It is not a coincidence that the very top of the top Jiu-Jitsu fighters don't fare well in MMA with a few exceptions like Jacare, Werdum, and Maia.
If you look at the way he is trying to apply the arm bar is that one: he didn't pinch his knees together. Look how far apart his knees are. The tighter you make things the less chance for an escape and the tighter and more secure the fulcrum is against the elbow joint (same with the Kimura when he should have held the elbow closer to his chest). Also, the thumb side of your opponent should be pointing up. This makes the elbow and wrist line up perfectly to dislocate. One of the escapes when getting caught in an arm bar is to twist your hand/wrist/arm outward like you are doing at bicep shot. This removes the straight line angle. And when doing this you want to roll over on your left outside shoulder (if it's your right arm you are defending) which I think is what Hardy did (I'd have to look at it again). This is my go to and first defense I always use when getting caught in an arm bar.
I was going to reply to Kwon, but you did here, with lots of detail.
I noticed you mentioned Werdum! He was a top tier heavyweight [or whatever the heaviest weight class was, if I'm not mistaken], and that is a good example you listed. I forgot how good Werdum was at Jiu-Jitsu.
I had no idea that Georges St-Pierre had a special invite to Abu Dhabi. I recall him having some off-UFC fight, but I didn't recall the part about him being submitted in the first bracket. I could have sworn he won something, but I must be thinking of something else. Maybe something from 2006. I guess GSP must have been competing unofficially. GSP hates to lose, so I guess it didn't count towards his record or something. I'll have to dig that one up.
GSP was big - around 185-lb at 5'10.5". I think this fight, UFC 56 against Sean Sherk was around the time you saw him. As Bisping said, clearly in pre-USADA times here, LOL. I feel like I could get to around this size myself, but I would need a little bit of Anavar - there is no way I could be that weight AND that conditioning drug-free.
I think on minimal gear, I could be around here. GSP is only a bit taller, but I can't imagine him being natural here:

pellius, do you have a view on how welterweights would do against any of the World's Strongest Man competitors? I heard that Eddie Hall passed the Navy Seals fitness test or something like that just the other day - except he didn't get the 1 mile run in 10 minutes.
My thought is that a welterweight UFC fighter or heavier would be able to beat them, but that a strongman has a chance in the first minute or two to do some damage.
I would like to see one of those hypothetical fight scenarios play out.
It's great that you know so much about combat sports. I feel like the UFC is just milking McGregor for some money fights but that's about it. I guess they can give him an easy fight a year, and keep the UFC pouring in money for a few more years. I don't see much else going in with the UFC right now, and Khabib wants to retire.
It sounds like GSP has said no, but how do you feel a 40-year-old GSP would do with a 32-year-old Khabib in Spring of 2021, if that took place at 155-lb?