You are correct. Even when finishing floor 100, you warp back around to floor 1.
I guess people saying "floor 0" could be used to describe people without a disk. Floor 0 could be thought of as the initial entrance lobby, and the task to complete is to click on the rune!
However, it is far more likely people use the term "floor 0" when really describing floor 1. For example, when you defeat the boss on floor 20, and either exit or go up a floor, you have 'cleared floor 20' but you are currently 'on floor 21'.
Things like this make the communication a little difficult for Nyzul. When you ask people which floor they want to do today, some people say "floor 100" and some say "floor 96" and their intention is the same. I believe the same thing is the case here. The only difference is asking what floor people are "on" and which floors they have "cleared".