Buddha taught dīghanakha-sutta to dīghanakhaparibajaka.
When buddha repeat dīghanakha-sutta to ānanda, and there maybe have something, that ānanda don't understand. Then he will ask someone such as buddha, sāriputta, etc. Because people, who are not in the same situation, can not know everything about teaching-situation, such as student's current mind, student' behave, etc.
The answers of the asking, is commentary.
(Notice: Most of sāriputta's books used linguistics as "question and answer").
So, when commentary describe "paripucchā (asking)" word from tipitaka (uddesa), they always describe as "atthakathā (commentary)".
That's why commentary said "commentary already bless in 1st saṅgayanā".
This is a simple thing, that whole tipitaka memorizer already known, because the history already became from the words in tipitaka. They don't need any explain, because the answer already on their, tipitaka memorizer, mouth. But reader always overlook the important words.
I have more information than I wrote. But it take a time to improve and to author them.
Last edited by theY
on Sun Jan 08, 2017 6:47 am, edited 2 times in total.