xinit suffers from an upstream bug, which does not allow non-root X to be run without a display manager via startx or xinitrc. Several people have reported this on #guix.

Currently if one runs startx as non-root, X reports:
(EE) parse_vt_settings: Cannot open /dev/tty0 (Permission denied)

This bug is further discussed in [1]. Redhat seems to have a patch [2], but it is unclear to me whether this has been applied upstream.

[1] https://bugs.launchpad.net/ubuntu/+source/xinit/+bug/1562219
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1203780