Using software to multibox is not against the rules. You have to be in control of the lead character and the other accounts perform the same action. So for example if you press 1, then the action for 1 is duplicated on all the other accounts.
People don’t seem to be able to distinguish boxing from botting. Botting is when no human interaction is required and the programme performs all actions for you. You don’t even need to be present.
The onus is on multiboxers to find software that does not break the rules, not everyone uses software to box either but the player has to be in control.