This was also unclear to me until this episode.
Riess described it as "obtaining the First King's knowledge of the history of the world." And made it sound like, once you know everything that's going on, you'll make the same decision he did.
But this episode presents it as "No, actually, the First King just possesses you, and forces you to keep enacting his original plan."
Of course, I guess there's no actually proof that the Scout's version is true and Riess' version was wrong. Maybe he was telling the truth and the scouts are just choosing to interpret it this way so they can feel good about their decision.