Suggest adding a USE flag for controlling whether the ebuild depends on xfce4-session or not. The package xfce4-session is an optional part in the package and the startup scripts shipped from upstream detect whether the session manager is installed or not. Motivation for configurability: it is the Gentoo way (it's about choice). The session manager makes the Xfce 4 startup time significantly longer that it is without that.
If you don't want xfce-session, then don't use the metabuild.