Chapman is correct that a person not knowing how close the windows were to the floor could reasonably assume that a kneeling person was standing.
However, in the case of the alleged sixth floor shooter, there were boxes in and by the window that would make it impossible to see a kneeling person "from the belt up" at the time of the head shot.
Rather than leaping to a conclusion that Brennan "forgot" what window he was talking about, I claim that it's more logical to suspect that Brennan -- a known serial embellisher -- embellished certain details after the fact.