金剛經纂要刊定記 4.304

4.304

入流者，四果名為聖人。經從凡夫剙入聖類，故流類也。預，廁也。

As for "stream-enterer": the four fruits are called nobles. Moving from ordinary beings, one first enters the category of nobles; hence a "stream" is a category. "To participate" (yu) means to join.