Thats what I got based on the lore, even the game mechanics reflect this, as your party easily decimates mercenaries just by virtue of your summons feeding on them and recovering and moving on the next group while orcs can't be fed upon and must be fought normally.
The bit about Kaalim being stronger than Aeolos is a bit of a surprise, as one succubus did manage to make Kaalim exert effort to beat even with a party, while Aeolos fought multiple succubi, granted it was a one on one fight.
How are the future humans losing to succubi, if their ancestors with swords and shields manages to fight them more or less evenly,? Succubus are a bit physically stronger than normal sword and shield soldiers, and have charm abilities, on a indiviual scale, but based on the leaf clan succubus showings, they won't be able to fight a organized military of the bronze age, let alone organized humans with modern weaponry, so how are future humans losing to the succubi even with advance technology?