Ah. I had assumed that processing the geometry and then displaying the image (along with any shader effects) fell under the umbrella of "rendering". Thanks for clearing that up.
As someone who has played the splitscreen mode numerous times with friends, I wholeheartedly agree with you. If they had just simplified the graphics (like in EVERY OTHER SPLIT-SCREEN GAME EVER, including Wipeout 64 and Wipeout 3), that wouldn't have been a problem.