I assume the point is that an individual wouldn’t just form a trust link with another individual without some confidence of their “true” identity. For me to trust someone’s currency, I would either need to be very sure of their identity through some trusted third-party system (e.g. a centralized system like government ID), or be involved in an extensive social (and probably in-person) relationship that would be difficult to fake (like friendship).

For the latter, what I mean is that it’s probably prohibitively difficult to maintain two significant in-person social graphs that have no overlapping members. Sure, people can get away with it for things like romantic affairs, but the incentive there is very different than money exchange, not to mention that romantic affairs often get discovered.