If a Qumran copy of Judges lacks a passage the Masoretic Text has, doesn’t that mean the passage was added later? That’s exactly the kind of corruption the claim denies.

It may mean that, and the variant is of real interest to scholars studying how the book took shape. The claim is not that no word ever changed but that the transmission was faithful and its variations are recoverable. A missing paragraph in one early copy, detectable by comparison, is a variant scholars can weigh, which is the opposite of an undetectable corruption. Faithful transmission includes preserving the evidence of where the text once differed.