I'm not sure if I agree with that. No, wait, I'm pretty sure I disagree with that.
If you're going to be buying stock art, then I think it makes a lot of sense to buy as early as possible, and to design the game around the art instead of the other way around. Because if you fully design the game before acquiring the art, then you're going to have a very hard time finding the art you need. The last thing you want is a full game with ugly placeholder art and no way of replacing the placeholder art.
If you're going to make your own art, then having some "real" art is a great way to experiment with and ultimately tie down the look of the game. It also allows you to quickly identify your limits as an artist. It's also best to tie down the look of your art as much as possible before you start animating, because it's going to be a lot harder to make changes once you've started animating. Do test your game, and your art, at all stages of development, but don't put off until the end. Often the art influences the game as much as the game influences the art.
On the other hand if you're hiring a third party artist to make custom art for actual money, then you probably do want to create placeholder art first, if only so that you can communicate effectively with your artist.