August renders a lot in his skins, actually. He exaggerates for effect when he talks about not rendering. He's explained before it's to encourage people not to spend too much time worrying about small details when the final skin is resized. The clean colour look is years of understanding how skins resize and general colourblocking skill. August is extremely good at utilizing shapes.

Here are some images he's posted in his skin shop with big images of his skins. There's clearly shading and rendering, but it's stylized. They're great images to study if you want to see how he works.

https://i.imgur.com/yRBJ0jI.png
https://www1.flightrising.com/forums/skin/3225785/148#post_58590104
https://i.imgur.com/ZNmaIIO.png
https://i.imgur.com/nfDskD6.png