> F off" has exactly zero semantic meaning

I don't think that's relevant. There are many ways to say no within few words - "No." is a complete sentence, "No thank you." is a polite one, "Get lost" has the semantic meaning that you want. etc.

The rest is not actually a reply to Mr Mosley, it seems more intended for other audiences such as us. Appeals to introspection not action, is not language that the fascists appreciate or even understand.

Don't get me wrong, there are many things to like about that thoughtful text. I just don't characterise it as "a succinct reply".