In normal use since auto brakes maintain a deceleration rate there is often little or no wheel brake action at higher speeds. If full reverse is used that will meet the deceleration rate called for at lower auto brake settings. As the aircraft slows and reverse thrust becomes less effective or is phased out wheel braking increases to maintain the same deceleration. What is a bit odd in this incident is they appear to have wheel brakes functioning as they went off the end. I have long forgotten my 737 systems so perhaps they were able to manually select a alternative mode.