Of course two very smart people can have a dumb kid and the reverse.
So what? The children of intelligent people are on average going to be more intelligent than the children of stupid people.
Why would anybody even try to argue this point?
Two tall people can have a short child, and two short people can have a really tall child, but on average heredity does its thing.
How does anybody think we developed dog and horse breeds?
Many (most) characteristics are hereditable.
One could, in theory, breed humans for desired characteristics (intelligence, beauty, height, etc.) just like any other species of animal. Ideas for doing so have always foundered on two rocks: the people in question won't cooperate, and the long breeding cycle of humans makes it impractical relative to other animals. (Not to mention ethical considerations. :))
That is what people are, you know, animals. We are not JUST animals, but that is what at root we are.
The problem is that people don’t usually select their mate based on intelligence. This includes “the rich” as well as “The Poor”.
This may enlighten:
https://www.youtube.com/watch?v=_BKPgI5lw_Q
BTW, I was basically parroting information I got from an article in Scientific American about 30 years ago. It was comparing sex with cloning and had a funny picture of “obviously dumb as a box of hammers” parents beaming over their daughter singing and dancing in her crib.