For the record, I don’t believe an omniscient being exists.
But omniscience isn’t disproven by describing a paradox. The paradox is observable and definable, and therefore knowable. This doesn’t disprove God any more than an Escher painting disproves architects.
The statement is only generates a contradiction if there is an omniscient being. If there are no omniscient beings, it is consistent.
The idea is that it is impossible for a being to both know and not know something. Knowable is not the same as known to a particular being
If there is an omniscient being, the statement is known. It’s internal inconsistency doesn’t make it unknowable. Omniscience isn’t bound by strict semantic logic.
Like, I couldn’t disprove you exist by saying “You don’t know this statement is true.”