Processed sugary junk food that passes for "breakfast cereal" 80-90% of the time bears little to no resemblance to anything humans anywhere in the planet evolved to habitually consume.
That is curiously true of everything we eat. Even carrots and potatoes. Today's hybrids don't resemble anything we evolved to eat, by a wide margin.
https://www.youtube.com/watch?v=Pc4zFD3vWHU
Calling horticultural changes in crop plants comparable to clumps of ground grain mass manufactured and loaded with sugar is a bit absurd.
When humans begin to cultivate a population of wild plants, there's a new selective pressure on those plants to better serve human beings. As a result, these horticultural practices in almost all cases created more nutritious food for human beings. Modern issues such pressure to look good on a shelf rather than keep human beings healthy enough to till fields are a blip on the radar, an issue stretching back less than 100 years. All in all the food we bred more closely matches what our evolutionary path lead our bodies to demand nutritionally, because if it didn't the plant's wouldn't have been able to outcompete their wild counterparts, people would stop growing them.
The watermelon specifically in the video was an underripe watermelon.
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.