An account should have a reason to exist
If the only job of the login is to hold your files hostage, skip the login.
An account is a tool. It can bill you, sync a machine you don't have, or prove you are allowed to see something that is not yours. Those are reasons.
A notes app does not need a reason like that. Neither does a grid over a CSV. If the work is a file on a disk you already own, the account is not a feature. It is a new owner.
Flatapps does not offer a Flat account, because there is not anything an account would do for you. No workspace to join. No family graph to migrate into. The folder is already the shared surface.
If a product cannot explain the account without saying “so we can save your stuff,” the stuff was never yours. It was a row in their ontology, wearing your filename as a costume.