I'm not sure what Phil refers to, but Radim-Gaudentius and Unger are two different people. Unger was apparently the one present during the "Akt von Gnesen":
https://de.wikipedia.org/wiki/Akt_von_GnesenThe doors were installed much later, in 1175. Maybe it's too late for me right now, but I can't seem to figure it out either. At first, I thought the author meant that the otherwise historically accurate scenes on the doors depict a bishop that shouldn't be there - it should have been Radim, who - according to Phil's link "accompanied Adalbert on his fatal journey to Prussia in 997." Yours reads: "aus dem Prussenland begleitet und einer ersten Grablegung des Heiligen." The Congress mentioned above was not at the same time.
Edit: On second thought, turn it around. Radim hadn't been Archbishop yet at the time of Adalbert's body being returned from Prussia, so it could have only been Unger. I am still not clear about which contradiction the author is hinting at. It's either that the scenes on the doors depict the wrong bishop or that people would've thought it was Radim on these doors, but actually it wasn't. Take your pick.
Best