I get from where you're coming from, but they were still pretty stiff even by Kpop standards when it comes to live audiences. They def weren't bad but there's still room for improvement.
I get what you mean, but I think their stiffness comes from over-preparation. If they could just remember it's just Coachella, not Inkigayo or Music Bank, they would have had more fun with it and let loose, I think.
But with that said, they were able to enjoy it more as time went on during the performance, so that's a great thing! They did well considering all that.
It's just a matter of mindset. They have what it takes in skills, talent, and training. Nervousness isn't too much to worry about; it's something they'll get over with more experience.