He doesn't have to be in RP mode. But as a member of Leaf's council, all players will respond to his attack as if it happened IC. And at that point it does become an IC event. You don't have to be in RP mode, because it's not the doers prerogative on whether he can wave it off as OOC or not...it's everyone else in the Shinobi world.
Espespially considering this: Additional orders: All killing missions to be focused on Missing Ninja or spars agreed to by Sand ninja
If our ninja attack the Sand first, they will retaliate rightfully.. - The Hokage 2017