Hard to say for sure but I have two possible theories for why Syla said that. 1) She didn't actually believe he wouldn't use it, it was just a comment to make light of the situation which seemed pretty central to her nature as she always made light of everything, never gloomy or serious. 2) Given that she called him "Indi", which sounds like an endearing nickname one might use for a friend, it's possible that Syla and Indra were actually very old friends that go back centuries or millennia but he gave himself completely to his patron so when Syla became a target of the 2nd, Indra could no longer resist or cared to...his identity was entirely consumed by his patron. This is something Ella warned the MC quite early in the game...she said the monsters want level 5 superhumans and to be prepared because they would come to claim him. Indra or this concept of being an incarnate is probably what that means...and the MC and companions will probably have to contend with their patrons trying to claim control over them at some point.This is intriguing, i wonder why Syla was so certain Indra wouldn't use this incarnation power.
My guess is that either the incarnation consumes the host, erasing their identity an leaving only the apostle, or because it's supposed to happen only when the apostle decides, so using at will would be against the wishes of Power.
If it erases the indentity then i'm starting to think that maybe Eye might actually be Body and the reason he never uses memory powers when he incarnates Mc is because he is not good at it.