Parinibbāna refers to a mode of consciousness that has been decoupled from its elemental ground. Think of it as a dimension of consciousness outside all relation, non-dependently arisen, non-intentional, neither objectified nor objectifying. The Buddha compares it to a beam of light that does not land on anything, is not traversed or impeded by an outside. The Zen insistence on realizing a "flavorless" consciousness (a mind that isn't labeled, reified, "colored", designated, etc. as anything other than itself by its relation with the world or its own desires), as well as Ajaan Mun's formula that the principle of suffering is a mind that seeks to go outside itself, proves it. Thanissaro Bhikkhu describes it as a mind that does not feed. Nibbāna is not some zero-point, but the mind of permanent, harmless satiation, which means a mind that is no longer compelled to appropriate its outside. I don't think this is crypto-eternalism because such a mind is inconceivable, and it isn't annihilationism because this state is endowed with qualities that we associate with the character of a Buddha. The mind is enriched, not diminished, by cessation. The Buddha did not make a full circle back to materialist death, because unconsciousness can't be identified with the deathless.

The kantposter is not wrong but he's not right either. His argument is that since knowing and the knower presuppose one another, there are no grounds to assume the noumenal existence of the latter. The point of Buddhist practice is precisely to cultivate the conditions by which the knower is eventually released from this relation of mutual positing.