I have checked the Flathub org repo and found none that had io.codeberg.username.app (except all had page.codeberg and I don’t know why)
as well as the domain ownership of codeberg.io, and currently nobody owns the io.codeberg domain, meaning that I should be able to use it?
Although, it does raise the question of whether if somebody takes up that domain, and then I will have to update the id everywhere.
See the Flathub documentation about which id you should use: