Esther’s absence might mean the Qumran community rejected it as Scripture, which undercuts the idea of a settled canon in this period.

That is one plausible explanation: Esther never mentions God and centers on a festival, Purim, that the Qumran community did not observe. It bears on the history of the canon, not on transmission, which is what this claim is about. Nobody here argues that the canon was fixed at Qumran, only that the books the community did copy were copied carefully. Esther’s later text is checked against the Septuagint and the Masoretic tradition instead.