Once you've really gotten the hang of a given PbtA or FitD game, I think it's true that it's very minimal prep. But I also think making the transition from running more traditional games to running PbtA/FitD can be surprisingly difficult and time consuming.
@John R Davis said upthread that you can play those games "wrong." I think you can absolutely run them wrong, too. There's so much you really need to reconfigure, about how a session might be paced (nearly always much, much faster than a trad game), how many NPCs you should be ready to provide names and info for, but maybe the biggest change you have to be prepared for is how much improvising you'll have to do, specifically to come up consequence after consequence.
To me, that was the most nerve-wracking part. If the average roll is a success with a consequence, how the hell do I constantly come up with interesting ways for a character to essentially fail while succeeding? But I had it wrong, because I was still thinking about consequences like critical failures (or similar) in other games. Like maybe you shoot the guy but the gun jams? Ok, but does that mean when the next PC also rolls a 7 to 9, they also jam, or they drop their gun, or something else that cumulatively makes the entire group seem like a bunch of clumsy dweebs?
It only made sense to me once I had read a ton of different games and listened to a bunch of podcasts (and peppered folks on here with questions), that consequences do not, and often should not, have to be PC fumbles. They can be something totally unrelated that increases the danger or stakes. They can mean you hear police sirens or a friendly NPC is hurt (not by a stray shot of yours, btw) or that, miles away, some enemy's plan is set in motion. Consequences just keep the whole engine moving--they're prompts for all sorts of improvised changes to the scene. And sure, sometimes they mean your gun jams or you hit a bystander, just not always.
So the prep, to me, is about reading lots of examples of consequences that other GMs have used, and sort of greasing your mental wheels, getting yourself ready to improvise the hell out of situation after situation, including by really familiarizing yourself with the setting and tone and related tropes. But once you have that front-loaded, overall prep done (and it's fun, imo, to reboot your brain to really do it) the individual sessions are definitely little-to-no prep.