P.S. I had made an error in my initial response. The following is the correct explanation.
You seem to be referring to the following sentence in the “Yasoja Sutta (Ud 3.3)”:
“Tena kho pana samayena bhagavā āneñjena samādhinā nisinno hoti.”
In the English translation there: “But at that time the Buddha was sitting immersed in imperturbable meditation.”
1. So, the translator translated “āneñjena samādhi” as “imperturbable meditation.”
– That translation is correct. It is the Buddha who was in this samādhi. Thus, āneñja, in this case, has the meaning of “no more rebirths”. Thus, it is the “Arahant phala samādhi.”
2. My confusion was due to the fact that cultivation of “āneñjābhi saṅkhāra” lead to “arupāvacara samāpatti” and the cultivation of arupāvacara samāpatti
result in rebirth in one of the four arupāvacara Brahma realms: ākāsānañcāyatana, viññāṇañcāyatana, ākiñcaññāyatana, and nevasaññānāsaññāyatana.
– I have discussed that in detail in the post, “Rebirths Take Place According to Abhisaṅkhāra”
3. The point is that “āneñjābhi” means “no more rebirths.”
– However, before the Buddha, anariya yogis had thought that the cultivation of “arupāvacara samāpatti” leads to Nibbana, i.e., no more rebirths. That is why the term “āneñjābhi” was used in the types of saṅkhāra (āneñjābhi saṅkhāra) that lead to those “arupāvacara samāpatti“.
4. However, the “Yasoja Sutta (Ud 3.3)” does not refer to “arupāvacara samāpatti” or āneñjābhi saṅkhāra.
– In fact, the single sentence in the sutta directly refers to a samādhi of the Buddha.
– Thus, āneñjena samādhinā in that verse refers to “Arahant phala samādhi.”
– Therefore, it is OK to translate it as “imperturbable meditation” because “Arahant phala samādhi” is “imperturbable.”
– Furthermore, there is no conflict with the material in the post, “Jhāna, Jhāya, and Jhāyi – Different Meanings”
– Please don’t hesitate to ask questions if the above explanation is not clear.