Tabula rasa says there are no real switches. Grandparent is saying that there are, but we don't have to obey them unthinkingly.
Not all switches in the human brain are for stuff that's as nice as caring for children or falling in love, so you might still want to think twice before unquestioningly endorsing their superior wisdom over conscious thought.
How does he know that we can override genetic behaviors? Sure, we can try, but that doesn't mean that we can override them.
Given that you could view large parts of human culture as semi-successful overrides of instinctual behaviors, I'm not entirely sure what kind of thing your thinking of that might not be possible even in principle. Not feeling the instinctual impulses to begin with as opposed to not acting upon them?
Not all switches in the human brain are for stuff that's as nice as caring for children or falling in love, so you might still want to think twice before unquestioningly endorsing their superior wisdom over conscious thought.