I would argue that since Instincts are formed from previous life experiences
The defies most definitions of what instincts are. Many inherited instincts are observed and pretty constant from culture.
things that took place in previous generations, after the people had been born, and because they found responses to certain experience were effective.
It's more likely that in the pre-modern world people without such instincts died. For instance there are a number of social instincts and reactions that have proven beneficial for a complex social species, people without such traits may become outcasts or less liked within the population meaning they were less likely to reproduce. These social instincts are such a part of human behavior that we say that people without them have a disability. One example I believe is Asperger's syndrome and varying forms of it.
Since these experiences were decision based, i.e., a choice in response made to circumstances like snakes and spiders, say, the instinct is a mental construct that appears in our mind."
They are not. The decision comes from breaking the "normal" behavior.
We KNOW something intuitively and instinctively which comes down to us and DIRECTS us from an ancient resouvier in our Unconscious mind.
We are directed yes.... but by behaviors often needed to be part of a population. Those that didn't have them in the past died or were less likely to reproduce.