X-ray to read cards?
I bet those were not X-rays in the sense which show your bones, but rather UV or similar invisible to human eyes. Which is enough to shine through the table fabric and then actually see the card. Literally.
'cause actual X-ray should not be able to see the card (even if you add lead-inlays), it just goes through, and without a plate on the other side you see nothing. And someone placing something on your cards "to get a good picture" would be weird.