Here is an account from nearly the very beginning of the Aṅguttara Nikāya commentary. Here you will see the legend told to you directly, that they were recited in the first council and brought to Sri Lankan and then Transcribed into Pāḷī. No secrets and to tell a blatant lie would just be unthinkable. I’m just baffled that people can accept this “citta” thing, and also count Chinese texts as an authority of the original Pāḷi when the Chinese have “Skillful Means” chapter that allows “lying”. Not only that, the Chinese texts were translated several times. The Account of the commentaries is listed below:
‘‘ekakadukādipaṭimaṇḍitassa aṅguttarāgamavarassa;
I shall elucidate the meaning of the excellent Aṅguttara Āgama, adorned with sections such as the Ekaka and Duka,
dhammakathikapuṅgavānaṃ, vicittapaṭibhānajananassa.
Which generates varied insights for the foremost Dhamma preachers.
‘atthappakāsanatthaṃ, aṭṭhakathā ādito vasisatehi;
The Commentary (Aṭṭhakathā), which was recited first by five hundred sages,
pañcahi yā saṅgītā, anusaṅgītā ca pacchāpi.
And later re-recited (in subsequent councils) for the purpose of clarifying its meaning,
sīhaḷadīpaṃ pana ābhatātha vasinā mahāmahindena;
Was then brought to the island of Ceylon by the sage Mahā Mahinda,
ṭhapitā sīhaḷabhāsāya, dīpavāsīnamatthāya.
And established in the Sinhalese language for the benefit of the island dwellers.
2‘‘apanetvāna tatohaṃ, sīhaḷabhāsaṃ manoramaṃ bhāsaṃ;
Now, removing the charming Sinhalese language from that old Commentary,
tantinayānucchavikaṃ, āropento vigatadosaṃ.
And rendering it into a faultless language that conforms to the Pāli Canon’s method,
samayaṃ avilomento, therānaṃ theravaṃsadīpānaṃ;
Without deviating from the tradition of the Elders, who are lights of the lineage of Elders,
sunipuṇavinicchayānaṃ, mahāvihāre nivāsīnaṃ.
And who reside in the Mahāvihāra with their extremely subtle discernment,
hitvā punappunāgatamatthaṃ, atthaṃ pakāsayissāmi;
I shall elucidate the meaning, abandoning what has been repeatedly stated, for the satisfaction of the good people and the long-endurance of the Dhamma.
sujanassa ca tuṭṭhatthaṃ, ciraṭṭhitatthañca dhammassa.
The descriptions of cities such as Sāvatthī, which I have given below
sāvatthipabhūtīnaṃ, nagarānaṃ vaṇṇanā katā heṭṭhā;
While explaining the meaning of the Dīgha and Majjhima Nikāyas,
dīghassa majjhimassa ca, yā me atthaṃ vadantena.
And the stories mentioned there in detail,
vitthāravasena sudaṃ, vatthūni ca tattha yāni vuttāni;
I will not discuss again in detail here.
tesampi na idha bhiyyo, vitthārakathaṃ karissāmi.
However, those meanings of the suttas that are not clear without their stories,
suttānaṃ pana atthā, na vinā vatthūhi ye pakāsanti;
For the sake of clarifying them, I will present the stories.