Red Choctober is kinda poorly-designed in that it's the only duel-related CE that's heavy on player-targeted mechanics and people standing in the wrong places can make it literally impossible to avoid taking vuln stacks.

The circle AoEs do go off slightly before the chocobo charges, but it's still possible to have them spaced in/around the safe zones in such manner that you won't have enough time to run in once they're gone.