functions.php

/**
 * Unspam Webmentions
 *
 * @method unspam_webmentions
 * @param  [type] $approved    [description].
 * @param  [type] $commentdata [description].
 * @return [type]                          [description]
 */
function unspam_webmentions( $approved, $commentdata ) {
	return $commentdata['comment_type'] === 'webmention' ? 1 : $approved;
}

add_filter( 'pre_comment_approved', 'unspam_webmentions', '99', 2 );