I feel like there's two different skill-sets at play--the ability to learn a piece, as printed, and play it back accurately, and then this, which is more improv

I feel like there's two different skill-sets at play--the ability to learn a piece, as printed, and play it back accurately, and then this, which is more improvisational skills. The two skill sets intertwine but mastery of one does not necessarily transfer. To learn the latter quickly, I find it easiest to build a catalogue of music you like listening to, and then figure out how to transcribe it (by ear) on your piano. Maybe just the most overt melody lines at first, before you can grok chords all that well. In time, you start building your own internalized library of "licks", little musical gestures that are automatic, and to achieve what Marc is doing, is to just string those gestures together. It sounds musically complex but when you're doing it, it almost feels like cheating, because you're leaning so hard on these "tropes" that you've built up over time.