A low alcohol result does not answer every question in an underage driving investigation. The video describes cases in which officers request urine testing after a result around 0.02 to investigate possible drug involvement.
An officer may suspect a combined effect of drugs and alcohol. A person under 21 may still face an impairment-based DUI charge, which is different from the zero-tolerance license process.
An underage DUI defense should identify which process is being pursued and what the evidence supports.