tattha viññātabbanti viññāṇaṃ nibbānassetaṃ nāmaṃ

viññātabbaṃ is the gerundive (future passive participle) of vijānāti meaning "to be understood, to be recognized, to be known." So the commentary seems to be saying, "Here viññāṇaṃ means 'to be known.' This is a name for nibbāna." The ṭīkā reads

Viññātabbanti visiṭṭhena ñātabbaṃ, ñāṇuttamena ariyamaggañāṇena paccakkhato jānitabbanti attho, tenāha ‘‘nibbānassetaṃ nāman’ti.

"It is to be known" - it is to be known by a superior person; the meaning is that "It is to be known perseonally by the highest wisdom, by the wisdom of the noble path," thus he says "This is a name for nibbāna."

I'm not sure where Suan is getting vinnyānaṃ in the sense of "it is known"? Perhaps he has another reading? The passive of vijānātiis viññāyati ("it is known") per the PED.

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Walshe translates as "Where consciousness is signless, boundless, all-luminous."

It seems to be saying that where consciousness, which has the nature of perception and discrimination (<  Skt. vi + jñā, "the act of distinguishing or discerning, understanding, comprehending, recognizing," Monier Williams), ceases its normal function and becomes signless, etc., - that is nibbāna. This seems to be consistent with the commentary which is equating viññāṇaṃ with non-discriminative (anidassanaṃ, "with no attribute" per PED) knowing,