I would guess his/her skepticism comes from the fact that Excel has about a billion + 1 features. Even Google and Open Office have trouble supporting everything. In that sense, I was too loose with my comment of supporting a superset. There's the old adage that everyone uses 20% of the features in Office, but it's a different 20% for every person.
Instead of "superset of Excel functionality" I should have said "superset of Excel semantics".
I doubt that.