This isn't correct. One example is that URIs don't necessarily provide the means of locating a resource. Another example is that the author says there are no URIs which are URNs.
The following example URIs illustrate several URI schemes and
variations in their common syntax components:
ftp://ftp.is.co.za/rfc/rfc1808.txt
http://www.ietf.org/rfc/rfc2396.txt
ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix
tel:+1-816-555-1212
telnet://192.0.2.16:80/
urn:oasis:names:specification:docbook:dtd:xml:4.1.2
I might be reading a different version of the article than you did, but the article agrees with you - URIs are a superset of URNs and URLs. I didn't see where the author says there are no URIs which are URNs - the article specifically says 'a URN is a type of URI with the scheme "urn"'
This is wrong according to the URI RFC, which the author cites: https://datatracker.ietf.org/doc/html/rfc3986#section-1.1.1