"I wondered how we could say these things about Mary — Theotokos and semper virgo."

Yet, the LCMS, and other Lutherans who claim a quia subscription to the Confessions, don't seem to say at least one of those things about Mary? (https://www.lcms.org/about/beliefs/faqs/the-bible#siblings)

At least one prominent LCMS pastor has argued that only the German version of the Confessions (or maybe just those of the Formula of Concord and Smalcald Articles) are authoritative and subject of the quia subscription. I disagree, but would be interested in the author's comment on these points.