I thought this wasn’t against the rules. I guess I’m not so certain about this then.
However, from my experience, the enforcement of this clause is lenient. Players have been scarcely banned, warned, or even reprimanded by staff because of teaming in general.
Furthermore, the main point of this rule is to dissuade giving any player a clear advantage in FFA, and again, team PvP in the duels arena avoids any disruption in the FFA arena (I’m not sure if the jurisdiction of this clause is implied to extend all the way to the duels arena).
Besides, team PvPing in the duels arena has been inadvertently supported. I’ve seen Madeira do it before, but perhaps his endorsement is irrevelant.