This recipe should be nice and thick, and it does get thicker as it cools.   But, if your strawberry cake filling isn’t setting or looks very thin and watery it could because the mixture was overcooked.  Cornstarch must be heated to a boil to set, but it can’t be cooked too long at a high temperature or it loses its thickening ability. If your filling is watery after cooking, make another cornstarch slurry by mixing ¼ cup room temperature water to 3 tablespoons of cornstarch in a bowl, add it to the filling, re-boil (for less than a minutes) and then continue simmering until thickened.