Very very rich people can pay for special early screenings. Disney "security" shows up and basically takes over the building to prevent leaks like this. (An AMC theater near my house did this for anonymous VIPs).
My guess is this was from a screening for a Saudi royal, and one their kids just snuck in a cellphone then started filming when no one was looking.
Disney is probably pissed about this, but the Saudis are so rich and influential there is basically no way to punish them. These guys probably paid a couple million to see this movie early, but, if yesterday's shitstorm is anything to go by, the reaction from fans worldwide will probably cost Disney a hundred million more.
Everyone loses here.