A partial faux amI - the “worst” kind to come to grips with in my view ! 

In French, in addition to the common English meaning of primordial,  ‘primordial’ also means vital, essential, or as Larousse puts it ‘qui est jugé de la plus grande importance’ or Robert ‘ qui est de première importance ‘. 

The use of ‘primordial’ is quite common in this context in French, and I have seen it used this way in other French teaching, but am yet to come across the synonym ‘prééminent’ in the same lessons - whether because I just haven’t read widely enough yet or because it is less used, won’t judge. 

https://www.wordreference.com/fren/primordial 

https://www.larousse.fr/dictionnaires/francais/primordial/63953 

https://dictionnaire.lerobert.com/definition/primordial