You're not arguing against the point. You are making it. If Anduin lacked the authority to sanction Genn for going against direct orders then Anduin lacks the authority to make peace talks between the Horde and Alliance since he cannot enforce it because, as you stated yourself, other Alliance leaders wouldn't be beholden to it. Genn attacked the Horde and faced zero consequences despite the High King's orders, therefore the Horde was justified in refusing peacetalks with a High King that cannot enforce his own orders.