The credential was a URL and nothing else, so the builder verified the broker
the ordinary way — against public roots. A mesh's broker presents a certificate
of the mesh's own, which is in no trust store anywhere, so the connection could
only ever succeed against a broker somebody else vouches for. It failed at TLS
with an error about an unknown authority rather than about a missing pin, and
the container sat there running: up, credential on disk, connected to nothing.
So the sealed credential now carries the URL and the broker's fingerprint —
the same two facts a node's token carries, for the same reason, delivered out
of band relative to the thing being trusted. The builder pins it: the standard
chain check is replaced rather than removed, and what replaces it is stricter,
accepting one certificate instead of every certificate a public authority
would sign.
A file holding only a URL still works, for a builder somebody runs by hand
against a broker with an ordinary certificate.