Most players, after dying a few times, would assume the KNIGHT's an unwinnable boss and skip over, but the shadow crystal folks would persevere and actually beat them, thus making it a secret that you can beat them.
Also, while external chapter foreshadowing is nice, the crux of the foreshadowing should be in your own chapter. Even in Tenna’s case where he was foreshadowed a lot in Chapter 2 you got a lot more of it in Chapter 3.
Ehh, doesn’t make the Knight’s fight itself a secret. Just the fact you can win.
yeah, and people might think its a predetermined outcome, thus not a boss in the sense that you overcome them more like an elaborate cutscene