The file picker isn't Nautilus; it's GTK. It's all library code, usually running in the same application.
One nice effect of applications moving to FileChooserNative (which they need to do if they want to be sandboxed nicely) is that for newer applications we can have that file chooser in a separate application (with broader permissions).
But even so, right now it's just the GTK file chooser in a separate application. In the future, Nautilus will be able to provide the file chooser, but that future hasn't arrived quite yet. Someone needs to build a file chooser mode for Nautilus, and someone needs to add a little more plumbing to xdg-desktop-portal so it knows how to deal with an application like Nautilus providing a default file chooser. But it will be much better, in part because the preview code will all be in one place :)
Maybe then you can use the KDE File picker on all your apps and not be forced to use the shitty GTK2/GTK3 one. I also dislike the GTK file picker limited sorting options(maybe was improved recently, I prefer sorting to be case insensitive)
One nice effect of applications moving to FileChooserNative (which they need to do if they want to be sandboxed nicely) is that for newer applications we can have that file chooser in a separate application (with broader permissions).
But even so, right now it's just the GTK file chooser in a separate application. In the future, Nautilus will be able to provide the file chooser, but that future hasn't arrived quite yet. Someone needs to build a file chooser mode for Nautilus, and someone needs to add a little more plumbing to xdg-desktop-portal so it knows how to deal with an application like Nautilus providing a default file chooser. But it will be much better, in part because the preview code will all be in one place :)