While trying to follow

Then I found a Gist that gave me a little more information here because someone commented that after they made the change, they needed to reboot their Mac (again): https://gist.github.com/cschiewek/246a244ba23da8b9f0e7b11a68bf3285

So, after I made the change AND rebooted my Mac, it worked!

Thanks for guiding me to the final answer!

ALSO NOTE: You do NOT need to volume mount the .X11 directory for this to work:

docker run -it -e DISPLAY="${IP}:0" so_xeyes