She does mention, and I will need to rewatch the cutscene for her exact wording, but something along the lines of primals being gods that once walked amongst men in Othardian legends. And indeed, there were primals present there in the past which the Garlean empire have faced.

Originally Posted by
Official Website - Solus zos Galvus
The eastern nations were the first to feel the newborn empire's mighty fist. It was during this campaign that the emperor witnessed firsthand the destruction wrought by the primals—an experience that led to an imperial mandate for their annihilation.