DocBook 5.1 as Normative Reference? Uncited in text

1.2 Normative References reads in part:

[DocBook 5.1: TDG] Norman Walsh. DocBook 5.1: The Definitive Guide.

However, DocBook 5.1: TDG isn't cited in any part of the text, other than in the citation.

BTW, DocBook 5.1 does appear on the cover page as a reference to this work so using ":TDG" is likely to be confusing.

It isn't clear why a non-standard text, which is subject to change and modification without notice, is being used as a normative reference.