Realm with stronghold as capital can secede their last city to create new realm.
Actually, they cannot. You cannot change allegiance or secede with the last city of the realm, even if the capital is a stronghold.
I think he meant it as a feature request.
If you wish to submit a feature request, please do so on the correct subforum (http://forum.battlemaster.org/index.php/board,30.0.html).
Please also make sure that you read the rules for submitting feature requests (http://forum.battlemaster.org/index.php/topic,2131.0.html) before you do so.