金剛經纂要刊定記 5.52

5.52

佛多下。出取喻之由。然說此經時，但在祇園，餘說法時多近於彼，故以喻也。

"The Buddha mostly, etc.": gives the reason for taking the metaphor. When speaking this sūtra, he was only in the Jetavana, but when speaking the Dharma at other times he was mostly near that [river], hence it is used as a metaphor.