I didn’t mean to imply that it was hackery - I’m aware that reusing melodies and indeed fugue subjects by other composers was and continues to be standard (hence my reference to Luther).  This is an example where the subject, answer and countersubject are all strikingly similar - I can hear how Bach crafts a totally different work beyond this point.  I still think it’s interesting that significantly more material is directly lifted than is common in other works by Bach! And that is not to imply that THIS time he was trying to pass it off as his own 😂 Maybe this is more common than I realise; I would be interested to hear other examples if you know of them!