This question was closed and migrated to Math.SE 20 minutes ago (as of this writing). User Marek posted an answer to this question 15 minutes ago. I thought question closure/migration locks the quesion from being answerable? How did that happen?
Closing does not necessarily block the question from getting answered, in that the server will accept answers but the UI generally tries to discourage this. See here:
Migration does, however, when the question is locked.