Steve,
The customer is correct, the only external ports are input only, seen below:

To connect another monitor, you would need to remove the top shelf, seen below. Just unscrew the black knob and pull the shelf towards you.

Then you would have full access to the back of the C20. Picture below is looking straight down after removing the top shelf.

You would then unscrew the back plates to fish the HDMI cable to where you needed it.

Justin