From the looks of Matt’s page he’s running a filter on either the img or gallery shortcode and replacing the caption with the comment count.
I’ve answered two related questions on how to modify the img and gallery shortcode functions, see.
How to customize the default HTML for WordPress Attachments
How to customise the output of the WP image gallery shortcode from a plugin?
What i think you’d be aiming to do here is selectively add a filter onto one of the shortcodes and change the caption text for the comment count(as it appears is done on Matt’s website).