I have trouble believing this, though I've heard it before. The watermelon in the painting looks exactly like the insides I've seen in my homegrown watermelons when things don't go right, i.e. under watered, not fully pollinated, or just underripe.

That's pretty normal for selectively bred plants - under stressed or unusual conditions it reverts to the older phenotype.

You have trouble believing the contemporary painting is accurate? Look at the linked art.