Well, I don't know what to say, except that it does work. I haven't done this with Dungeon World in particular, but I've played and run Apocalypse World games online (also played Dungeon World once and Monsterhearts once), and that model is really perfectly fine.
We can establish tone in discussion, and I can nail down any themes that feel like they need to be stated. That's the important part, that prevents things like that stumbling into a gritty war story.
At the same time, the group builds characters and makes bonds, and I direct questions to the PCs. When I have a bit of a grasp on the party, I set the first scene and go from there. This is pretty much per the First Session chapter in the book, which in turn cribs a fair bit from Apocalypse World.
It's a proven technique, and you really can trust it, even if it does sound a little loosey-goosey.