A professor who studies watermelon breeding disagrees with you
To check, I contacted professor Todd Wehner, a professor at North Carolina State University who studies watermelon breeding.
At first glance, the photos look a lot like the painting. But the Stanchi painting gives us a clue with its black seeds, which Wehner says indicate the melon was ripe:
"In the painting, the black seeds indicate that the fruit has reached maturity," Wehner says. "If they had waited longer for harvest, the fruit would have continued to break down, the flesh would have gotten softer and stringier, and the sweetness and redness would not have improved much." A melon that wasn't ripe wouldn't have those black seeds.
"Museum paintings are an interesting method for studying old cultivars [varieties], and the one you indicated certainly shows the sort of watermelons that Europeans had to eat in the Middle Ages during their summer harvest season," Wehner says. "We have cultivars like that one in the painting available to us now from our germplasm collections [a sort of genetic sample library that includes many different varieties]."
He notes that those samples, when grown today, have "large white areas, low sugar content, [and] frequent hollow heart." Hollow heart can cause a starring appearance somewhat similar to an unripe or underwatered melon