As raised in the IDPF forum[1], the ONIX ISBN identifier 15 code was not 
updated when the examples were changed from number strings to URIs.

The correct code should be 22, although as noted if this were an actual ONIX 
record the usage would not be correct. I've verified with editeur that they 
don't see the usage here as problematic, since the package metadata is not 
being used in a trading context.

It was also noted separately that there currently isn't a 13-digit URN scheme 
for ISBNs. The document in which the scheme is defined is now an expired draft. 
Usage isn't wrong, but is perhaps not optimal.

The "urn:" scheme identifier should also be removed from the doi: example, as 
noted.