I see X_5[1] = X_4[0] - X_4[1] here as well as in the description, but it is X_5[0] being changed from X_4[0].

Aha, this is where the typo in the description comes from ;) It should be

X_1 = [6, 9, 21] # -> X_1[2] = X[2] - X[1] = 21 - 9

I have updated the proposed Haskell translation with this as well.

Thanks for better detailed description, just updated kata