If your application is inside a sandbox, it communicates with another process that presents the file chooser and hands over permissions based on the user's selection.
Plenty of Flatpak applications actually do use this, but the author of this website loves to pick out the ones that don't so he can act like the project is flawed to the core and justify his sensationalist domain name. But actually it is very solvable, it is being solved, and in many cases you can tighten an application's sandbox with a two line diff.
If your application is inside a sandbox, it communicates with another process that presents the file chooser and hands over permissions based on the user's selection.
It is possible to choose directories in the same way, and portals are always improving: https://github.com/flatpak/xdg-desktop-portal.
Plenty of Flatpak applications actually do use this, but the author of this website loves to pick out the ones that don't so he can act like the project is flawed to the core and justify his sensationalist domain name. But actually it is very solvable, it is being solved, and in many cases you can tighten an application's sandbox with a two line diff.