Lüdemann also argued the disciples hallucinated. You can’t borrow his authority for one conclusion and dismiss it for another.

Nobody is borrowing his authority wholesale. Lüdemann is cited here for one narrow, checkable point: a scholar who spent decades trying to explain the resurrection away never found it plausible to deny that Jesus lived and was crucified. His hallucination theory is a separate question, and it is weighed on its own merits in the Resurrection of Christ case. Agreeing with an expert about a fact while disagreeing with his interpretation of that fact is ordinary reasoning, not a double standard.