In addition to the pitted cherries, the pie filling also contains a great deal of brown sugar, starch, and a shredded apple, all of which contribute to the gelling of the filling. (The apple contributes pectin but is otherwise not noticeable in the baked pie.) The Elsens’ recipe also includes a couple dashes of Angostura bitters, although I suspect you’d have to run a side-by-side blind tasting to tell what difference it makes.