金剛經纂要刊定記 4.227

4.227

佛問等者。空生疑得疑說，佛即順疑以問。辭雖云得，意顯無得，試其所答解與不解。

On 'The Buddha asked…' and so forth: Subhūti doubted attainment and doubted preaching; the Buddha then followed his doubt to question him. Although his words spoke of 'attaining', his intent was to reveal non-attainment, testing whether his answer showed understanding or lack of understanding.