The Document Style Semantics and Specification Language tells jade how to render a SGML document into print or online form. 14 point Times Roman for RTF, for example. It merely changes the way the rendered text looks. The DocBook DTD defines the tags and structure of a DocBook SGML document. Modifying the DTD, such as adding a new tag, doesn't make it DocBook anymore.
Some applications require jade, but are being updated to support either software package.
This is the successor to the sgmltools project, which has officially been disbanded for over a year. Since then, Cees de Groot has created a slightly different project, which acts as a wrapper to the jade SGML processor. It hides much of the ugliness of syntax. This author was able to install the old sgmltools package followed by the sgmltools-lite and could format this document quite easily. There's even a man page for sgmltools showing syntax.