Probably only effects discussion's DateInserted, not comments.
Here's a workaround that I just came up with that reinserts DateUpdated and DateLastComment into the new discussion that is created after the WP post is published. It seemed to do the trick when I tested it several minutes ago.
At the end of function glue_add_discussion($postid) in glue.php:
// Workaround for incorrect DateUpdated and DateLastComment timestamps after the discussion is created on Vanilla.
// Reinserts DateUpdated and DateLastComment to the newly created discussion on Vanilla.
$wpdb->update(VANILLA_PREFIX.'Discussion', array('DateUpdated' => $the_post->post_date, 'DateLastComment' => $the_post->post_date), array('DiscussionID' => $DiscussionID));