The watermelon was cut to eat on a feast table. All indications are, it was the best they had.

Fantasies about 'eating paleo' and such are just that, fantasies. To eat like we did thousands of years ago, you'd be eating plants that were barely digestible with toxins and organic compounds designed to injure us. Because until we tamed them, the plant had no pressure (as you say!) to be nutritional.

No, boxed candy cereal is not good for you. But eat oatmeal then (which is a cereal). Which is digestible, because we modified it heavily.