Shubhat al-maḥall addresses uncertainty attached to the legal status of the subject or the entitlement involved. In some historically discussed cases, law recognizes such an uncertainty even where the actor’s own assumption points the other way.
This is why individual belief and an objectively recognized legal basis of doubt are not interchangeable. The existence of the latter does not always depend on the actor's subsequent statement.
The ownership and kinship illustrations belong to a premodern legal setting. They explain a technical doctrine; they do not prescribe or justify contemporary slavery.