Please add <http://wayland.freedesktop.org/xserver.html>.
$ grep -n wayland /usr/portage/x11-base/xorg-server/xorg-server-1.17.1-r1.ebuild 16:IUSE="${IUSE_SERVERS} glamor ipv6 minimal nptl selinux +suid systemd tslib +udev unwind wayland" 74: wayland? ( 75: >=dev-libs/wayland-1.3.0 173: $(use_enable wayland xwayland)
Oh, interesting. I'm not entirely sure I agree with that... but thanks for the info!
xwayland is built by the wayland USE flag on xorg-server (just as the link you gave says).