I think -bhuto can also refer to the past. AN6.55 for example says, Soṇa, kusalo tvaṁ pubbe agāriyabhūto vīṇāya tantissare”ti?, "Soṇa, in the past, when you were a layperson, were you skilled at playing the lute?”
If the adverb pubbe wouldn’t be there, it seems to me the sense of this sentence would still be the same, just like we can leave ‘in the past’ out of the English translation and the meaning wouldn’t really change.
We also wouldn’t translate this literally as “having become a layperson”. That is not the idea, because Soṇa was a layperson from the moment of birth until he ordained. Literalism isn’t the answer here, and it almost never is. Usually it’s the worst approach imo. It’s the context that tells us what is going on, not the literal words themselves.
Of course, when the context is meditation, the problem is that it is very subjective to the translator. In the case of evaṃbhūto both interpretations seem grammatically possible. That might well be on purpose, for the same “refrain” also has to work for the reflection on enlightenment (the third celestial bed), which seems possible to do while walking. Either way, to insist the grammar has to be interpreted one way or the other just to make a point about what jhānas are like, feels to me like clutching at the tiniest straw. Even more so because in the very sutta itself the Buddha says he sits down before he attains jhāna.
We can interpret evaṃbhūto literally it as “become thus”, with the implication being that he has exited jhana before he “became thus”. This exiting jhana is implied all the time in the suttas without being explicitly mentioned, e.g. SN28.1:
“Reverend Sāriputta, your faculties are so very clear, and your complexion is pure and bright. What meditation were you practicing today?”
“Reverend, quite secluded from sensual pleasures, secluded from unskillful qualities, I entered and remained in the first absorption, which has the rapture and bliss born of seclusion, while placing the mind and keeping it connected. But it didn’t occur to me: ‘I am entering the first absorption’ or ‘I have entered the first absorption’ or ‘I am emerging from the first absorption’.”
The implication is that Sariputta had this conversation after coming out of jhana, but that is never mentioned explicitly.
The same thing is going on with the Buddha walking after “becoming thus”. That’s how I see it.