Yes, that's a legitimate bug. I don't actually recall if the toolchain that built the (awful) XSD versions was ever able to produce the XInclude version.
I can try to create an XInclude version of the "by hand" XSD files. However, that will be a version 1.1 XML Schema. Is that good enough, or do you need 1.0?
[Addendum]
After some investigation, it appears to me that the UPA rule in XSD makes creating an XInclude version enormously difficult. Simply allowing XInclude at either the division level (part or reference) and the component level (preface, chapter, et. al.) violates the UPA rule because a book can start with either a division or a component.