This is a common statement, but one that isn't considering that the difference is in how you go about doing play. That there's a large difference in outcomes and impacts. If you only get to explore your character in relation to the things that the GM provides for you to engage with (and, if you're lucking, the GM has taken some of your suggestions and written them into their setting or story) then that's going to be a world of difference from a game where play is centered on interrogating the character -- who they are, what they want, what they're willing to do to get it -- and where play isn't anodyne to character.
A good example would be to consider a WotC AP, like Curse of Strahd. This AP doesn't care what characters show up to play it -- nothing about the structure of the plot, the locations, the NPC motivations, the point of play, etc., changes with a different party. However, while you play you can represent your character in the situations provided. However, this representation is going to be reflective of the provided play -- it won't be informing it. There's no analog to the Curse of Strahd AP in narrativist play -- the entire concept doesn't make sense. There aren't published adventures for strongly narrativist supporting games. Setting is very thin (Stonetop, which has a beefy for PbtA setting, is mostly prompts for consideration when something comes into play, meant to be used only as impetus and then only as it bears on and is informed by the current play). This is because everything is going to be centered around the characters. And not all of it will be player authored -- the GM has a role to play as well, the provider of honest adversity towards the PCs.