Polenta is technically ground corn, just like cornmeal. However, to be considered polenta, it is typically made from a specific variety of corn and often ground a little more coarse than cornmeal. But yes, you could make this recipe with cornmeal, though I would recommend a medium or coarse grind for the right consistency. You may have to cook it a bit longer. Just follow the directions on the package, and don't skip the step of letting it rest covered in the pan and chilling in the freezer.