They absolutely look at on field performance. No one has said otherwise. They absolutely do not care about the stats. As has been shown, year after year, draft after draft. If they see a guy who is running good routes and has the speed and gets open all the time and his quarterback can't get him the ball, or his team just doesn't throw much, or he misses time injured, or whatever, they don't care that the guy doesn't have the numbers. They care that he can play. Similarly every year there are guys with gaudy stats who don't get drafted high, and in some cases don't get drafted at all, because NFL GMs don't care that the guy put up big numbers, they care if he is going to be able to play at the next level.
A GM who drafts based on stats isn't going to be a GM for long, because his drafts are going to be awful.