you want multiboxing fixed in pvmp? then make pvmp instanced
It looks like they have plans to add small pvp battles, so perhaps pvmp instances will be introduced and they will have to deal with that problem as well. They can also have the right to interpret their rules as they see fit, for example, update it as in the Guild Wars 2 where multiboxing is allowed for pve with some exceptions:
Multi-boxing may ONLY be used in PvE areas. You are not permitted to run multiple accounts simultaneously in PvP*, WvW, activities, or in other competitive environments.
* We make an exception for the use of multiple accounts in custom arenas, but encourage anyone hosting a custom arena to, as a courtesy to all participants, specify that multi-boxing is allowed in that arena.
They are gradually fixing the aspects where it had a reputation for being a game with a lot of bugs with free interpretation by the players, sooner or later it will affect pvmp otherwise why bother updating it as a whole. The potential contingent of players doesnt stop at multiboxers, most new players simply will not play such competitive content, including pvp, if they consider it as a foul game. Obviously, in this case, the opinion of multiboxers doesnt matter for the company, there are many times more ordinary players.