I'm going to refer to the Battle Myth brain as June.
June having arceus would mean that she would have a pokemon that is banned, and thus she couldn't realy be it could she? All legendarypokemon should be allowed, even ones banned from other facilities.
The sentence right before that says that all legendary Pokemon are allowed in the Battle Myth, INCLUDING any that are banned from other facilities. This means Kyogre, Mewtwo, Jirachi, Deoxys, Palkia, Darkrai, Arceus, and every legendary in between is allowed.