To *truly* know someone? Do you somehow know their dreams and hopes by reading their visual gestures? Do you read their history and feel their pain?
Well... if we're going to be strict, each second of the day we change who we are due to the slightest environmental changes and the chemistry that results from it, so as soon as you know someone completely (were it even possible) you'd lose it a split second later. So it isn't possible in it's absolute; knowing their every memory and stored sensation, without being them. I'm aware of this... and I don't mean knowing their memories/etc, but knowing the impulses and rhythm of their nature. Why the attack?
It's actually quite offensive to tell people that you "truly know them"
It may be offensive to you, but you can't speak for everyone.
...when you actually know nothing about them.
I do. But it's not upbringing-specific knowledge. Just like a scientist can know a lot about me by the fact that I'm hispanic (things I don't know about myself like the percentile of african, etc, i have, or disposition to diseases). You seem to be viewing the only type of relevant knowledge as being knowledge of their life circumstances/chronology/history. That isn't the only dimension of knowledge belonging to a person.
I think you can know a lot about a person by knowing their psychological type. I've observed many times through the years that people who have the same type tend to gravitate to the same likes, dreams, occupations, habits, etc. It may sound like sacrilege to individualists, but we are born as one of many copies of our mind. Everyone is different in small ways due to specific familial genetics and especially outward circumstances, but imo nature has a much larger impact on the personality of an individual than environment.