How would you react if someone came in and said "there is no real gravity; it's just that the earth loves us very much"? Or "there are no viruses and bacteria; we get sick from bad air and evil spirits"?
That's more or less the level of wrong that deepstack is. The Big 5 Personality Inventory is probably the most robust, reproducible piece of psychological science we've got, and while there might be arguments that it needs more nuance in places, the bare existence of introversion and extraversion as traits is not even slightly in question in the scientific community.
Your feeling that it might not be true—or that I'm "overly confident" in its truth—just doesn't hold up to decades of scientific consensus.