If you search for something like "ancient garden drawings" you get a lot of pics going back hundreds of years that include circular elements with a little path into the middle. Although I think a lot of these are for formal landscape architecture type gardens, not vegetable gardens. Here is a collection of such illustrations:
https://www.pinterest.com/nidzalina/ancient-garden-drawings/
Since the keyhole garden is a sensible, intuitive design that uses the most basic of shapes, I'm going to guess that keyhole gardens have been independently "invented" by numerous gardeners since the dawn of agriculture.