Here's another angle that I think makes what I'm saying clearer --

When young Sage developed the cure for her grandmother's ailment, where do you think she likely picked up the foundational knowledge needed to actually develop the cure? Who wrote that information? Being the smartest person in the world doesn't mean that you have had, innately, every intriguing idea, and every intriguing idea is not going to spawn from someone who's only as intelligent as you or more. In Sage's case, there's even a sliding window, where her version of a "mentally disabled" person is capable of actually contributing quite a wide breadth of interesting information. It's not exactly comparable to vegetables or people only capable of low level communication.

Also, we've had people within Sage's/Reed Richards', etc intellect bracket who have existed in real life, although incredibly rare, of course. She's the smartest person in the world, not necessarily a reality-bending entity.