no more themename_comment for drupal6?

from reading the drupal6 api, there is no more defaule theme_comment implementation.

but on line 1580 of common.module:
$output .= theme('comment', $comment, $node, $links);
it actually calls theme_comment.

the problem is the older version(5.x) of the function is defined like this:
function theme_comment($comment, $links = array())

newer version has a new argument $node. this little creature gave me a few days of headache.

I wonder why comment module still makes a call to theme_comment. :(