The most obvious conclusion is that a human made 500,000 shirt designs?
It's an obvious thing to do. Amazon already sell print-on-demand ebooks, allowing vendors to spam the products list with a torrent of shit at minimal cost so they can find out which bits stick. This is exactly the same principle.
(See also, in case you're reading this later and there are a lot of comments: https://news.ycombinator.com/item?id=5310567)