Richard Wallis posted:

[In the linked data world] there is a significant difference between
the >numbers (OCLC number, LCCN, ISBN, etc.) associated with a
resource and the >URI that identifies it.

Of these numbers, only ISBN is associated with the resource.  The OCN,
LCCN, and other national bibliographic agency nubers, are associated
with the description.

That is true, the numbers have been associated with the records (descriptions). However the URI is a [linked data] identifier for the resource. 

Note the '[In the linked data world]' in the text above you referenced.  Linked data uses http URIs as identifiers for resources, so that they can be linked and those links followed.  Obviously there is need to record numbers and other identifying strings (which are not http URIs) that have been used to identify the resource in other domains, as properties in the RDF description.

ISBN is a bit of a special case, it is an identifier for the resource, it is a string, it is not a http URI that can be used as a linked data identifier.  So in RDF it is captured as a string property. 

~Richard.