This is not a type of rdm,
Since you've rejected to duel him again in open world, he still able to claim his revenge, since you've won your duel vs him, but still brings the reason that you've attacked/killed him
as that was his last option, as you've rejected dueling him again, The revenge is clear; there's nothing with this.
Ch0ppa.
If the ramming was on purpose or not, you shouldnt have done that way; it can still be punishable, but I'm advising you to not repeat the same thing over here