https://cp-algorithms.com/dynamic_programming/knuth-optimization.html
In the proof of the lemma, this line:

Using the QI on $C$ and on the dp state for the indices $z+1 \leq y+1 \leq c \leq d$ (from the induction hypothesis) yields the desired result.

should be:

Using the QI on $C$ and on the dp state for the indices $a \leq b \leq z \leq y$ (from the induction hypothesis) yields the desired result.