Configures, compiles and installs fine without docbook-xml-dtd
Correction: Compiles fine with app-text/docbook-xml-dtd-4.5-r1, so the dependency on 4.3-r1 seems unnecessary.
I you have internet access, this will indeed work as the DTD will be transparently downloaded for validating/transforming the man page. However ebuilds are meant to be run within restricted environments so the dependency is required. The exact version is mandated by the source file of the man pages.