金剛經纂要刊定記 1.84

1.84

由斯者因此也。即正指說此一卷經，是密是玄也。此則結指前文之所明，標為後說之所以也。

"Due to this" means because of this. It specifically points out that the exposition of this single scroll of the sūtra is esoteric and mysterious. This concludes and indicates what was clarified in the preceding text, marking it as the reason for what is stated afterward.