You're right. The Archbishop of Trier at that date should be Dietgold although he was excommunicated and stripped of the position by Pope Nicholas I on October 31 that year.
The position was vacant until 869 when Charles the Bald got his candidate, Bartholf, elected to the position. At this point, Trier was basically Charles vassal.
Anyways, this situation could be in place to simulate these events.