You don't need to be able to completely understand something from the great cosmic womb of unbeing to be able to feel a connection to its destructive aspects and piece together reasons why it is dangerous; people fear and invoke the Abyss regardless of its unpinnable nature. The creatures of the Abyss manifest in ways that fit into the sensory framework of experience regardless of their deeper incomprehensibility, because they manifest in the Phenomenal World — the Begotten truck in ominous phenomena does not cease to have a point of reference just because there's an ultimate unknowable on the other end of the line.
Making them not-kin has more consequences than just disqualifying them from the more active Kinship abilities, like rendering a manifested gulmoth as unremarkable to Begotten senses as an unmanifested one. It's a blind spot that doesn't have the same motif of disconnection and compartmentalization and mundanity that the Machine does to justify cutting an entire class of supernatural entity out of an explicitly nigh-universal sense for the immediate presence of supernatural entities. It's one thing to put kinship with "the Abyss" off the table (you might as well try to claim Kinship with "the Wyrd" or "the Shadow") and another to cut out "any Abyssal entity."