Do units that ignore terror (bretonian knights, demons, flaggelants and so on ), also count attackers as terrifying?
By the letter of the rules - yes, they do, if affected by this spell.
The key sentence is:
If the unit would normally cause terror then it ceases to do so and counts all enemies as terrifying just like everyone else.
IIRC the 'causing terror' automatically means 'immune to terror'. So, such super units may be affected by this spell like
everyone else excepting Undead and Daemons (Booo! It's magic!).