This sutta and Commentary is rather complex, yes.
here is Sujato’s translation along with the Pali of the section:
And what fuels the arising of the awakening factor of tranquility, or, when it has arisen, fully develops it?
Ko ca, bhikkhave, āhāro anuppannassa vā passaddhisambojjhaṅgassa uppādāya, uppannassa vā passaddhisambojjhaṅgassa bhāvanāya pāripūriyā?
There is tranquility of the body and of the mind.
Atthi, bhikkhave, kāyapassaddhi, cittapassaddhi.
Frequent rational application of mind to that
Tattha yonisomanasikārabahulīkāro—
fuels the arising of the awakening factor of tranquility, or, when it has arisen, fully develops it.
ayamāhāro anuppannassa vā passaddhisambojjhaṅgassa uppādāya, uppannassa vā passaddhisambojjhaṅgassa bhāvanāya pāripūriyā.
and the Commentary:
Kāyapassaddhīti tiṇṇaṃ khandhānaṃ darathapassaddhi.
“Calming of the body” means the cessation of agitation in the three aggregates [feeling, perception, volition].Cittapassaddhīti viññāṇakkhandhassa darathapassaddhi.
“Calming of the mind” is the cessation of agitation in the consciousness aggregate.
I find it reasonable that body in this case means the body of mental factors as to me mental tranquility is prime (see also Ceisiwr’s posts about this in the thread).
Take the case of sitting very still, and sure anyone can do that. Years ago I used to go along to a Zen-do near a beautiful lake. And I was quite pleased that as a ‘Theravadan’ I could sit still for 2 rotations while the Zen aficionados had done sitting then walking and then back to sitting. How pleased I was when the organizer praised me for that.
But was there any tranquility in the sense of the sutta above - well that depends entirely on the mental factors at the time. Actually I could be sitting peacefully and be immersed in a range of kilesa (defilements) - no different really than any other time.
Back to why should we accept the Commentary on this and other points. Remember that Buddhaghosa - the writer of the Sāratthappakāsinī (this Commentary) - had at his side the ancient Commentaries many of which date back to the time of the Buddha. He edited them and his work was recognised and accepted by the great monks at the Mahavihara. They are an integral part of the Theravada.
I think even someone who reads all three from cover to cover - and even if with a noble attainment - no one can be absolutely certain that there are not deviations here and there. By ‘in agreement’ I mean I am certain that the Commentaries never knowingly attempt to refute or change the meanings of any suttas - so we might find any discrepancies can only be minor and likely to be from our side. Regarding the Abhidhamma, the Theravada hold it as the word of the Buddha - he first taught it to his mother and the myriad devas - so of course it is in agreement.
Although there are little areas where no one knows. The Commentaries are open about such matters. They might say “according to the Digha reciters … . But according to the Majjhima reciters …” (I paraphrase).
There is for example the case of Mogallana in a past life. According to the Dhammapada reciters he killed his parents. But the Jataka reciters (or another group of reciters) says he intended to kill them but relented after the first blows . He was however reborn in hell in both cases. And when he took his last birth the residual kamma of the act meant he could be killed by the band of assassins.
It should also be noted that one could read the entire Sutta Pitaka and come to wrong view:
The Atthasalini (translated as the Expositor
p31)
The bhikkhu, who is ill trained in the Sutta , gets a wrong idea, not knowing the
meaning of such passages as, ‘ There are, bhikkhus, four
persons in the world,’ concerning which it has been said,
‘ Owing to his wrong ideas, he accuses us, harms himself and
produces much demerit.’ Consequently he arrives at wrong
> views.
And also the one who studies Abhidhamma can go wrong:
The bhikkhu, who is ill trained in the Abhidhamma,
makes his mind run to excess in metaphysical abstractions
and thinks of the unthinkable. Consequently he gets mental
distraction.
However all of us, even before noble attainments and without reading the entire body of work of the Theravada can begin to understand the truths right now. The texts pertain to life as it really is. In the sutta we are discussing, another section says
“And what, bhikkhus, is the nutriment for the arising of unarisen doubt and for the increase and expansion of arisen doubt? There are, bhikkhus, things that are the basis for doubt: frequently giving careless attention to them is the nutriment for the arising of unarisen doubt and for the increase and expansion of arisen doubt.
And this can be seen to be true, if/when doubt should arise.