Canonical could have done a lot better with the explanation message here. The idea is to push apps towards XDG compliance and the use of things like Portals.
That said, unlike Wayland, portals really aren’t there yet from a UX perspective, especially for an app that is heavy on file transfers.I prefer what Flathub does where it puts a nice green checker beside your app for XDG compliance - it’s an encouragement, but not an enforcement.