Story is just an overall 'idea' of what is going to happen and who the primary characters are. It's usually very broad and doesn't entail too much detail when telling another person what is happening. Screenplay is the written word put down on paper which includes dialogue, scene descriptions, beats, etc....This is where all the details are fleshed out. Whether or not they actually sat in the same room together to knock this out....who knows. My guess is that Spielberg called Chris up over the phone and said "hey, I have an idea. It's about a bunch of kids going on a literal treasure hunt for buried gold all in the effort to save their house. Along the way, they get have to navigate their way through obstacles, etc...... Is this something that you're interested in? I'm sure that once they agreed to work on this they probably had more in depth conversations during the process of hammering out the script.