After being able to examine a Divi theme, seems that it does not use the default wordpress function to show the featured image nor does provide a way to hook into the one it uses. So you’ll have to edit the theme code (or better make a child and edit the relevant files) to make it work
i.e. in the single.php file of your theme you can find something like
print_thumbnail( $thumb, $thumbnail["use_timthumb"], $titletext, $width, $height );
there are to options, one is to simply replace that function with the wordpress default one:
the_post_thumbnail();
the other is to use the functions provided by our plugin to get the caption text:
print_thumbnail( $thumb, $thumbnail["use_timthumb"], $titletext, $width, $height );
get_featured_image_caption(array('tag'=>'p'));
In most situations, you’ll prefer to use the second one, as it allows you to better customize how the caption is shown while maintaing the original divi configuration for the image (size, classes, etc.)
Hope that helped you