Nonfree license for SGML?

For the following files:

/usr/share/doc/doc-base/version.ent
/usr/share/liferea/dtd/html.ent
/usr/share/sgml/X11/defs.ent
/usr/share/yelp/dtd/isoamsa.ent
/usr/share/yelp/dtd/isoamsb.ent
/usr/share/yelp/dtd/isoamsc.ent
/usr/share/yelp/dtd/isoamsn.ent
/usr/share/yelp/dtd/isoamso.ent
/usr/share/yelp/dtd/isoamsr.ent
/usr/share/yelp/dtd/isobox.ent
/usr/share/yelp/dtd/isocyr1.ent
/usr/share/yelp/dtd/isocyr2.ent
/usr/share/yelp/dtd/isodia.ent
/usr/share/yelp/dtd/isogrk1.ent
/usr/share/yelp/dtd/isogrk2.ent
/usr/share/yelp/dtd/isogrk3.ent
/usr/share/yelp/dtd/isogrk4.ent
/usr/share/yelp/dtd/isolat1.ent
/usr/share/yelp/dtd/isolat2.ent
/usr/share/yelp/dtd/isonum.ent
/usr/share/yelp/dtd/isopub.ent
/usr/share/yelp/dtd/isotech.ent

I found this license:

     (C) International Organization for Standardization 1986
     Permission to copy in any form is granted for use with
     conforming SGML systems and applications as defined in
     ISO 8879, provided this notice is included in all copies.

Does this prohibit making a non-conforming SGML system? Sounds nonfree.