The article states they were rendering maps screens on the phone and shipping the screenshot to the watch to handle performance issues with the watch.
Even if your architecture is so hosed that you are screencap'ing the actual screen to get an image to ship over a network connection … multiple people thought that tradeoff with security was worth it?
No, but you do need the ability to render in the background, and apps aren't allowed to do any GPU-based rendering in the background (you can't touch an OpenGL context, and while I haven't actually confirmed this I assume you can't touch a Metal one either). This entitlement probably let them skip that restriction to do fast rendering in the background.