金剛經纂要刊定記 6.122

6.122

不增減者。即釋經中無有高下。謂在聖不增故無高，居凡不減故無下。斯則平等之義也。

"Neither increasing nor decreasing" explains "there is neither high nor low" in the sūtra. That is, residing in the noble it does not increase, so there is no high; dwelling among the ordinary it does not decrease, so there is no low. This is the meaning of equality.