For many decades leftists pushed the blank-slate theory of intelligence. That is the "theory" of intelligence that has been totally discredited. Intelligence is not all nature, but it is a very large part of it.
That most people are smart or dumb due to heritable IQ is something libs are just not ready to admit. To admit that would mean ideological defeat and the end of all useless lib programs designed to equalize outcomes and waste tax-payer money.
It seems to me that intelligence is much like height.
Every child is born with the genetic potential to grow to a maximum height or IQ.
We know perfectly well how to change the environment to keep the child from reaching that maximum potential, but we have no clue how to help a child born with low height or IQ genes to grow past that point.
Which makes all the talk about “equality” more than a little creepy. The only way we presently know how to achieve it is to stunt the high achievers.