Hi Bhante, thanks for your reply. 
Perhaps I mischaracterized the text in that way before, but I did consider your opinion, and when I said “their practice does not lead to enlightenment” I actually meant their whole system, the practice being whatever complete path they taught, not just the practice of the formless meditation. Sorry, I should have made that more clear. Because I agree, it is clear that if the Buddha did indeed attain the formless attainments under his teachers, as the text do say, he did not reject these states per se. He taught them many times. So then he must have rejected something else.
But that still leaves me with questions. If it was just their theory which was wrong, why did the Buddha not realize before his awakening, “Ah, right view (or vipassana or whatever) is the path!” Instead he specifically said he now realized jhana is the path to awakening. (MN36) If what was new to his approach was not the samādhi but the view, this statement to me is just out of place. The way I read it, it also feels very much like the jhana was contrary to whatever else he had tried during his quest, including whatever he did under Alama Kalama and Ramaputta.
To me it also doesn’t answer why he recalled the jhana of his earlier days, not the meditation under his former two teachers. I can’t agree with your assessment in the other thread: that what was different about his earlier attainment of jhana was that it wasn’t couched in theory. Because we’re never told this anywhere, nor are we told what theory Alara Kalama or Ramaputta would have taught, only that they taught the formless attainments. Again, if the theory is what made the difference, why did the Buddha not focus on that? Both the former-teachers story and his account of how he did discover enlightenment seem to focus on samadhi, not view. His recollection while his father was working also states nothing beyond the standard jhana formula. And if right view was made all the difference, when sitting under the Jambu tree he apparently had no right view either, so that’s doesn’t seem to be the point. (PS: For those interested, in the sutta it never says he was a child at the time. I think that’s from the commentaries.)
And it also doesn’t answer why, if he indeed was so proficient in the formless attainments to the extent he was asked to lead his former teachers’ followings, he still feared the jhanas later.
In the other thread you say:
If it’s a narrative detail, then perhaps in reality he didn’t actually attain these states. Let’s assume his former teachers were indeed brahmins. What if he just understood the theory? Because for brahmins theory or conceptual knowledge was generally deemed enough, as we tend see in the Upanishads (yo evaṃ veda). Then to me the story would make sense.
To perhaps support this a bit, the Buddha does not literally say that under his teachers, “I entered the state of nothingness”, or “I entered the state of neither-perception-or-non-perception.” He just said he realized that dhamma that his teachers declared. His conclusion that these dhammas only lead to rebirth in those states was made before his enlightenment, so could simply have been wrong. In other words, his teachers taught rebirth in such a state based on a certain theory, but it wasn’t really the case, and they didn’t really attain the real formless states.
To put another engineering theory out there. 
. Much more impressive to use the story of childhood jhana, which emphasises “I discovered (rediscovered in SN12.65) the way myself, not because of my teachers.”
Beginner’s Mind! Excellent! How did I miss that connection before? 
Hear hear! Let us not lose sight of the trees. 