Was that using an RNG? Or is the entire game deterministic?

It used an RNG. The usual practice back then was to spin a counter while waiting for keypresses, so that might affect the question when dealing with an external harness, I suppose.

I feel like you are being pedantic. There are very few parts of Zork that are not static to the game. Yes the thief shows up randomly but that’s not the main point of the game.

It is not the least bit pedantic. Games were meaner back then. If you're on a time (turn)-limited section of the game, or in a vulnerable spot like the volcano, random encounters with the wizard could render the game unwinnable without dying, which would completely wreck a benchmark. Same for the thief in Zork 1. If he randomly steals your light source, you're done for. Or if the RNG dictates that you lose the fight with the troll.

Can't recall anything like that in Zork 3. (Edit: apparently you could get shot randomly when using the time machine in the Royal Museum.)