But it’s not true. Consider by analogy: if you can’t explain something to a 4-year-old, you don’t understand it yourself. After all, you were a 4-year-old once yourself.
No, actually, sometimes you can’t explain something to someone because you don’t have a good enough understanding of their mental processes. It doesn’t matter if you once experienced those same mental processes; the relevant memories of that time are very likely lost to you now. Explaining math to novices is a different skill than understanding math. It requires the ability to figure out why the other person has got it wrong and what they need to hear. That isn’t a mathematical skill.
A distinguished math professor is probably inferior at explaining arithmetic to 8 year olds than an experienced mathematics educator, but it doesn’t mean the latter has the better understanding of math. They just have a better understanding of 8 year olds.
I don’t understand why it’s supposed to be somehow better to have more people, even if they are equally happen. 10 billion happy people is better than 5 billion equally happy people? Why? It makes no intuitive sense to me, I have no innate preference between the two (all else equal), and yet I’m supposed to accept it as a premise.