A single copy tells you nothing about stability. There’s nothing to compare it to except the medieval text you’re trying to validate.

That is the situation for Ezra, and nothing more than present in the collection is claimed. Comparing one early witness to the medieval text is still a comparison across a thousand years; it just cannot detect variation within the Qumran period itself. The claim as a whole rests on the well-attested books such as Isaiah, Psalms, and the Pentateuch. The thin ones are listed so that you can see where the evidence is thin.