The newly-added introspection files in libxfce4util-4.13.0 are only installed to the system if --enable-maintainer-mode is used. This happens because the whole 'if HAVE_INTROSPECTION' block in Makefile.am is inside 'if MAINTAINER_MODE'. I suspect this wasn't intentional.
Eric Koegel referenced this bugreport in commit cfd26455a732d955509e0e0f75d0b6b741e23ca7 Allow everyone to build introspection files (Bug #13638) https://git.xfce.org/xfce/libxfce4util/commit?id=cfd26455a732d955509e0e0f75d0b6b741e23ca7
Thanks for pointing that out! Marking closed.