I never played Zork, but on the MUDs that I have played (Viking Mud, Midnight Sun, Deeper Trouble) there were some ASCII 'graphics'. Perhaps this is what is meant?
Interesting, I've played exactly the same MUDs. Scandinavian fellow, eh?
Zork is interactive fiction though (or text adventure as it used to be called), not a MUD. It's completely textual.
I think the later games (maybe Zork 4) had graphics after INFOCOM sold off the IP. Something along those lines maybe. I've only played Zork 1 and that was text only.