Wait, so Sakuya already knows that the calamity is caused by the convergence of the swords being gathered together, because it's why he's going out to destroy them. And he knows that the Holy Rose knight order has gathered a bunch of holy swords, which is why he's trying to get to S Rank.
...should it really be so surprising to him that the people gathering holy swords are the people who cause the calamity, caused by gathering swords? Given, they only gather holy swords, and not magic/cursed swords (publically at least), but I feel like this shouldn't be something he "never" expected. They should've immediately been on his radar.