well, is it?

Kind of, yes. The lobby is called a "Square", and its a little town where people interact, go to shops, talk to npcs and find quest, etc. And you can join other rooms called, "Stages" and thats where people fight monsters and such. You can make your own private stage as well. There are also PvP rooms too.

It's kind of like room-based but not quite. Besides entering the gameplay itself by clicking on Start and choosing a stage where you will join and others that click Stage around the same time will join the same stage as you are, you can enter a particular stage by entering a party. So it kind of like a room-based but not so direct.