That's false. You can in fact make a PC that is not below average anywhere. 10 points will give you straight 10's, and then you have 17 more points to raise stats from there. 15, 15, 11, 10, 10, 10 is 27 points, as is the example 13, 13, 13, 12, 12, 12 which is explicitly above average. So 12 by RAW is not high average. It's above average. Similarly 9 is below average. Average in 5e is 10-11.

I have no reason not to believe you, but your degrees are irrelevant to whether or not you can roleplay a low intelligence. And your still trying to conflate low scores with high scores. They aren't the same. You can easily roleplay a score lower than your own, but will find it impossible to roleplay a score higher than your own.

And the modern definition of roleplay is simply, playing a role. That's it. Acting is only one method of playing a role.