I've worked enough with electronic medical diagnostic equipment to know that movement artifacts can play havoc with trying to get good readings. Terri's behaviors, even if merely "reflexive", are inconsistent with a flatline EEG. Dr. Hammesfahr in an interview offered an explanation for the flatline EEG which seems plausible and consistent with my experience (Terri was fidgety, and motion artifacts were swamping everything else; turning down the gain to eliminate the motion artifacts eliminated everything else).
Any doctor who would accept a 'flatline' EEG from someone who moans when prodded ignores one of the major rules of medical diagnostic equipment: if the equipment's indications contradict clinical observations, don't trust the equipment.
You're making an incorrect assumption.