I think the truth behind the SY sword may initial occur as a surprise, at least to me.
But in retrospect, it may be a resolution to Chang Whui’s conflict between compromising to the nobles(, which isn’t what he wants to do,) and abandoning Gil Dong(, which he isn’t willing to do either). Since Chang Whui turns out to not be the crown prince after all, so the need to please the nobles doesn’t seem as essential anymore because that support for the rightful king is no longer present. He is then free to truly become a king for the people, supported by the people.
In that sense, the twist to the story isn’t an awkward insertion of some unpredictable-ness near the end to steer up some excitement, rather it’s a nice resolution and character development device. Hopefully I’m right.