I read once that it was a demo that Bruce Springsteen did for the producers of the film in his home studio, and that it was an Oberheim.
That's going from memory, and zero research.
Yeah it´s a sparse arrangement and quite some demo productions were released as final mixes (after additional mixing/mastering). I recall, that for example Celine Diones Titanic theme as we know it was the demo with the vocal scratch takes. As always, performance is the key.