“Is there any reason to distinguish between antisemitism on the one hand and the persistent excusing or minimisation of antisemitism, on the other?”
Being of a pedestrian practical mind, I can offer one practical reason to distinguish between the two. Where anti-Semite should be treated by energetic application of a blunt object, a Jooish apologist of the said anti-Semite (like Mr Kuper) deserves no more than a slap (followed by a spit) in his face.
Purely a matter of priorities, you see…