Maybe WitnessX is secretly controlled by WitnessY

And you wouldn't consider that a form of Sybil attack? To me, that's the very definition of it, and it's not a matter of scale which determines the identity.

IMO, very few systems (if any) are sybil-resistant. Some are wide open to it, and some have some protections which can still be bypassed under extreme or unusual circumstances. Maybe it's better to release a false assumption than to claim this wasn't a sybil attack. The term, to me, is still very useful, regardless of scale employed because the identifying characteristic of a pseudonymous identity is the problem. We want distributed, decentralized systems, not centralized systems pretending to be otherwise.