What section 10.2 actually says is:
| For public libraries intended for use by other packages, these files
| normally should not be included in the Debian package, since the
| information they include is not necessary to link with the shared
| library on Debian and can add unnecessary additional dependencies to
| other programs or libraries. 6 If the .la file is required for that
| library (if, for instance, it’s loaded via libltdl in a way that
| requires that meta-information), the dependency_libs setting in the .la
| file should normally be set to the empty string. If the shared library
| development package has historically included the .la, it must be
| retained in the development package (with dependency_libs emptied) until
| all libraries that depend on it have removed or emptied dependency_libs
| in their .la files to prevent linking with those other libraries using
| libtool from failing.
The .la file is used by the shipped xapian-config script to support more
esoteric (but handy) use cases, so I've followed policy here by
emptying dependency_libs.
| If the .la must be included, it should be included in the development
| (-dev) package, unless the library will be loaded by libtool’s libltdl
| library. If it is intended for use with libltdl, the .la files must go
| in the run-time library package.
And it's included in the -dev package as per this part.
Cheers,
Olly