I side with the passengers - the guy did tweet that there were at least 10 doors open. Assuming all one side in his carriage were open, would you stand and approach a vestibule where the draft is unknown & may suck you out? I'm not sure I would....now if those alert buttons were in the seating areas and NOT in just the vestibules, easily reached.
Yes, and there are even stronger reasons too. Stopping a tube train is a tunnel is never a good idea, as it makes whatever you do next more difficult and take much longer. Worse, while there is a small risk a passenger may go near the doors and fall out while the train is moving, there is a much larger risk someone will try to get out once it's stopped. I'm not sure how big the clearance is in Jubilee Line tunnels, but it's still a bad idea to go wandering off into it.