The politicians have access to much better data than they used to. Given Biden showed a similar drop in grade level, I suspect what this is secretly revealing is that most voters didn't understand what politicians were talking about for most of the USs history.

Without exactly joking, or exactly being serious, democracy isn't about voters understanding the real issues. Just the person capable of figuring out how to get the voters on side is also canny enough to make good decisions when they get to the real issues. Most people neither know nor care. The debates are moving in the direction of that world - theatre to judge how well politicians do theatre, and that is the proxy of their actual ability. It isn't possible to catch up on what matters about someone in 3 political debates.