Sorry I only skimmed this when I originally read it as I was more focused on hearing your points in your own words. But now I have read your source, and it is not what you said. That quote you gave is not about the Buddha’s teachers. It is about “some recluses and brahmins”, and I have suggested above that they may well be simply hypothetical ‘some recluses and brahmins’ rather than actual people. But if we are to take it literally, it does not even imply that they are from any time in the past, but rather that they may be contemporaries. But whether we consider them hypothetical or not, let’s consider whether your quote means jhāna practice is involved. I assume you may consider it is due to the part that says:
some recluse or a brahmin, by means of ardour, endeavour, application, diligence, and right reflection, attains to such a degree of mental concentration that with his mind thus concentrated, [purified, clarified, unblemished, devoid of corruptions], he recollects his numerous past lives
I would like to know what this author means by his use of square brackets - whether that was in the original in abbreviated form, or whether the translator added on an assumption. Does anyone know?
Well here is Walshe’s version of what I assume is the same passage in DN 1 - no square brackets in this one:
'Here, monks, a certain ascetic or Brahmin has by means of effort, exertion, application, earnestness and right attention attained to such a state of mental concentration that he thereby recalls past existences
Let’s see DN 28 (Walshe translating), where we have this we have this:
Here, some ascetic or Brahmin, by means of ardour, endeavour, application, vigilance and due attention, reaches such a level of concentration that he considers just this body upwards from the soles of the feet and downwards from the crown of the head, enclosed by the skin and full of manifold impurities: " In this body there are head-hairs, body-hairs, nails, teeth, skin, flesh, sinews, bones, bone-marrow, kidneys, heart, liver, pleura, spleen, lungs, mesentery, bowels, stomach, excrement, bile, phlegm, pus, blood, sweat, fat, tears, tallow, saliva, snot, synovie fluid, urine." (as Sutta 22, verse 5) That is the first attainment of vision. Again, having done this and gone further, [105] he contemplates the bones covered with skin, flesh and blood. This is the second attainment.
I don’t know if the Pāli is the same and he was just in a different mood (and I don’t have time to check), but even if the Pāli is different, the meaning is apparently almost identical. But I do not see any direct evidence that this was connected to jhāna practice. It seems more like mindfulness of the body parts practice.
So I ask, are you sure that your source is referring to jhāna practice? If so, what is your reasoning?