金剛經纂要刊定記 1.90

1.90

但云一真者，但猶獨也。以聞說此經是空無相宗，則首末作離心離境空無相道理一味銷釋，故云一真。

As for "merely speaking of the single true": "merely" (dan) is like "solely." Because they hear it said that this sūtra belongs to the emptiness and markless school, from beginning to end they uniformly dissolve and explain it according to the principle of markless emptiness that is apart from mind and apart from objects; hence it says "the single true."