Thanks for the feedback! But I’m afraid, that explanation isn’t based on the usage in the early suttas. Matsumoto is an expert on later forms of Buddhism; but my interest is early Buddhism, and that is what concerns us here. No scholar of early Buddhism would cite Fausboll these days. His translation was published in 1881, and we can do much better than that!
In the suttas in general, the imagery and usage clearly shows that the sense of nibbāna or the past participle nibbuta is “quenching, extinguishment”, as is supported by virtually all commentaries and modern scholars. K.R. Norman, regarded as the leading linguist of ancient Indic dialects, always rendered it as “quenching”. This is also the only meaning given in Cone’s Dictionary of Pali, the latest and best authority on the meaning of Pali words.
It’s true that the exact etymology of nibbāna in Pali is a little complex, as it can appear to show a heritage from more than one Sanskrit root. But in usage the meaning is straightforward, as for example one of the classic passages from MN 72:
19.11 “Suppose that fire burning in front of you was extinguished. Would you know:
“Sace te, vaccha, purato so aggi nibbāyeyya, jāneyyāsi tvaṃ:
19.12 ‘This fire in front of me is extinguished’?”
‘ayaṃ me purato aggi nibbuto’”ti?
Since the meaning is well established in the Pali texts themselves, the opinions of Tibetan or other commentators are not really relevant. As you know, they normally relied on much later texts and interpretations, as few of the early texts are available in Tibetan. Commentators and the like are useful when there are no clear contexts that give the meaning; but that’s not the case here.
As a point of method, it’s best to avoid relying on poetry to establish meaning, as words in poetry are frequently used in obscure or unusual ways, or, as here, in ways that do not clarify the meaning. Wherever possible, we establish meaning in clearly defined, straightforward cases, then infer from there to the more obscure contexts.
In the verse in question, the line should be translated something like:
A mendicant is peaceful, quenched inside themselves
It has nothing to do with the ātman as understood in Hinduism.