I’m pretty much with your definition, though I have heard people calling games with “player authorship” rather than or as well as “GM authorship” components storygames (eg: Fate, PbtA stuff, Cortex+). Still, I prefer your version. Things like Microscope, and Duty and Honour fit perfectly.

Where I strongly agree is in seeing Storygame as a subset of RPG. There may be edge cases (is Microscope really an RPG?), but I don’t care. These things are descriptors rather than taxonomic classifications, right?