Oh, I'm sure that's the reason. Her recipe calls for more liquid in the filling than ours, so it will be comparatively looser. The recipes we have just use, molasses, water and baking soda. The dry ingredients also absorb some moisture during baking helping the filling set.
There is a similiar pie, Montgomery pie, that uses an egg in the filling. Montgomery pie adds lemon to the filling and has a more cake like topping. It's not as widely available as Shoo fly pie? Perhaps, because the ingredients were historically less readily available, particularly in frugal Plain households.