We all existed before we were born, even Jesus. G-d knew us all before we were born.
Looks more like a shabby translation. Probably should have been 'was' instead of am.
I would say that God knows we are going to be born because he's outside of time. But within time we don't exist before we are born. Adam didn't exist before God formed him. We don't exist until we are formed by God in the womb.