Yeah that's not a clear answer and sounds like they may have been confused by the question. It seems they're thinking multiple copies of the game on one account, rather than multiple accounts, which would be how you would do it in this game as it won't let you be logged in on 2 separate characters on the same account and once in this game.

I expect it's a grey area thing where if you do it and use it as an exploit somehow then it'd fall under the ToS as an 'exploit', which against the ToS.
But if you're not using it for that purpose then I think it's fine, I don't see anything forbidding it. If you're not doing anything dodgy, then they get 2 subs.

If a third party tool is used then yeah that falls under their 3rd party clause. But in this game I see a limited practical used for dual boxing, for any battle content it'd be a really difficult task, not like XI where dual boxing was great for power levelling because the game had the pacing for it to work. Otherwise, I can see RP benefits, social benefits maybe, crafting benefits maybe (for convenience), transferring items between characters, possibly lower levelling and undersizing benefit, creating identical characters and making them hug so it's really narcissistic, confusing people yada yada.