I gather by the mention of fascism that the correspondent is a bad person. So it makes sense that Russell told him to get bent. But, that is all that he's really saying here.

I can only guess this is noteworthy due to the parties corresponding because it isn't very interesting outside of that.

Have you been reading the news? Perhaps about someone who engaged people in debate while holding extreme views? In the process, they gained some measure of credit amongst people with less radical views, merely for the act of having conversations. Except in this case the debates were not with Bertrand Russell, but with 18 year old college freshman.

I understood the posting to be a subtweet-style comment on that.