Well, this is a slightly difficult passage, and I’ll just try to draw it out a little here.
The operative word is evaṃbhūta. This is a compound of two of the most widely used and flexible words in Pali, evaṃ meaning “thus, such, in that sort of way”, and bhūta, the past participle of “to be”, i.e., “become”. Obviously it’s a somewhat idiomatic usage. Unfortunately, there don’t seem to be a lot of other uses of this term, so it’s a little difficult to tease out the exact implications. Let me first begin by mentioning a few possibly relevant contexts.
The closest related term is in the contemplation of corpses, where one reflects that I, too, “will become like that” (evaṃbhāvī). Here there is a future orientation.
In Sanskrit, the dictionaries give the sense of “of such a quality or nature, such”.
The commentary to AN 3.63 defines it thus:
Evaṃbhūtoti evaṃ paṭhamajjhānādīsu aññatarasamaṅgī hutvā
Evaṃbhūta means: having thus been in possession of one or other of the absorptions, starting with the first, etc.
I think the main point of this is to explain that not all four jhanas are necessary, but any one is enough to count. Note, too, that the commentary affirms the past aspect of the term, rendering the past participle with an absolutive indicating a past perfect.
In the Pali, evaṃbhūta only seems to occur in one other passage. This is closely related to AN 3.63, except not in the context of jhana. This passage occurs at AN 4.11 and in a few variations elsewhere.
“Carato cepi, bhikkhave, bhikkhuno uppajjati kāmavitakko vā byāpādavitakko vā vihiṃsāvitakko vā.
“Mendicants, suppose a mendicant has a sensual, malicious, or cruel thought while walking.
Tañce bhikkhu adhivāseti, nappajahati na vinodeti na byantīkaroti na anabhāvaṃ gameti, carampi, bhikkhave, bhikkhu evaṃbhūto ‘anātāpī anottāpī satataṃ samitaṃ kusīto hīnavīriyo’ti vuccati.
They tolerate it and don’t give it up, get rid of it, eliminate it, and exterminate it. Such a mendicant is said to be ‘not keen or prudent, always lazy, and lacking energy’ when walking.
Here the usage is more straightforward, and we can render it more easily as “such a mendicant” or “a mendicant of such a kind”, etc.
Note that the grammatical form of the word inflects the meaning subtly, in ways that may or may not be easy to capture in translation. In the corpse contemplation, with its future orientation, we could say “I will come to be of such a kind”. In the past tense, “I have been of such a kind” or perhaps “I have come to be of such a kind”. Whether this aspect should be rendered in translation is a subjective point.
Anyway, so that’s a bit of background on the term, although it doesn’t directly solve the problem. Given that the term is vague and rarely used elsewhere, we shouldn’t overburden it with meaning. Following my usual “principle of least meaning” we should avoid creating dramatic or difficult meanings out of simple or ambiguous terms.
I think the overall sense of the passage is something like, “At a time when I have been practicing these meditations, when I walk, stand, sit, or lie down, it is just heavenly!”
It reminds me of something one of the lay people in Perth said to me one time. He’d been on a meditation course with a visiting teacher. This wasn’t a residential retreat; not even a retreat really, they just gathered to practice each evening. Many of the students were telling the teacher that they’d been attaining this jhana or that jhana. But the teacher never confirmed any of this, just taught meditation.
The lay student couldn’t help being skeptical, seeing how the yogis were going to work each day, chatting about this and that, talking about food, and generally being normal people.
Previously he had been on a long retreat in Burma. Even though he was a long term meditator, he remembered one time that just blew him away. He had such a profound sense of peace and stillness, a deep absorption, that his mind was completely changed after. For days, it was as if he was walking around on a cloud, everything bright and still and pure. Such is the power of deep meditation.
And that, in my view, is what this sutta is talking about.