How so? As far as I understand, it will hold to a very high degree as long as the flight conditions are stable over the time scale needed to establish the airfl

How so? As far as I understand, it will hold to a very high degree as long as the flight conditions are stable over the time scale needed to establish the airflow pattern over the wing, which is very short. (If the flight conditions are that unstable, a mechanical AoA vane will struggle too, since it has mechanical inertia.)