4 Conformance reads in part:

The reference documentation describes additional constraints and processing expectations. A conformant DocBook V5.1 document should respect those constraints and anticipate those processing expectations.

Err, there is no "reference documentation" referenced in the conformance clause.

By way of anticipation, DocBook 5.1 would not qualify as "reference documentation" because unlike standards it is subject to change without notice and doesn't clearly separate normative from non-normative requirements.