It's usually healthy skepticism borne out of the fact most of the Vietnamese VAs available in the US would have learned Southern Vietnamese. Plus the narrator uses the southern ch when calling Trieu by name (Chiêu?). I.e. ty/ť/ть [c] instead of ch [t͡ɕ].Why would he think she speaks with a southern accent when she's wayyyyy before when that becomes a thing?
Though I don't think that will be the case here myself.