> Or for some industries back channel checks in network

Even in small-world industries, assuming they occasionally accept outsiders, they will still encounter some form of this problem.

>assuming they occasionally accept outsiders

I guess it comes down to industry. We're on hn so emphasis is on technical ability and in that context what you say is true. I'm in a space that requires trustworthiness is part of the core value proposition so there is little acceptance of outsiders and much emphasis on back channel checks that the candidate is solid. NK fake candidate etc is just not a thing in that context

This happened to me where an interviewee used me as a reference (not a good idea!) and the interviewer knew me and called to verify.