@ref reads http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-att.canonical.html
(reference) provides an explicit means of locating a full definition for the entity being named by means of one or more URIs.
change to
(reference) provides an explicit means of locating a full definition or identity for the entity being named by means of one or more URIs.
The example given is a URL to VIAF, which doesn't provide a definition, it provides an identity, as do all authority control systems, which appear to be a common use case for this attribute.