The startup script of Xorg changes /etc/sysconfig/Xserver which is used to indicate the current Xserver, the one that exittc to uses to exit from.
Typically, I do not try to install Xorg from within any other Xserver, instead, I exit to prompt and use
ab Xorg
Early versions of exittc used a shotgun approach and tried successively Xvesa, Xfbdev, and finally Xorg. But such approach limits and is specific to the above collection.