🚀 go-pugleaf

    I start xdm from /etc/ttytab on my Sun 3/160 (running 4.0.3) as
    follows:

Don't do that.  xdm isn't intended to be run this way; xdm runs in
background by default, and this will cause init to complain.  You
should simply start up xdm like any other daemon, from your /etc/rc.local
(or whatever):

if [ -f /usr/bin/X11/xdm ]; then
	/usr/bin/X11/xdm; (echo "xdm")			>/dev/console
fi

    One way I came up with to get rid of these errors is to have xdm
    invoke the X server with its (the X server's) standard error
    redirected to /dev/null.

The X server isn't generating these, the clients are.  I doubt what
you said really fixed this, or doubt that what fixed this is really
what you said.  xdm logs output from the "session" to the xdm-errors file.
If you really don't want this, have your Xsession redirect these
somewhere.