The user_get_user_lastaccess_sql function (user/lib.php) has a sql table alias parameter, that is not used in the sentence:

return $tableprefix . '.lastaccess != 0 AND u.lastaccess < ' . $accesssince;

Same with user_get_course_lastaccess_sql (user/lib.php):

return $tableprefix . '.timeaccess != 0 AND ul.timeaccess < ' . $accesssince;

Both functions are only called from user_get_participants_sql using the default $tableprefix value, therefore they go unnoticed.