Ah, I had missed the ambiguity. There is a potential double meaning in: "Mr. Stace says that my writings are 'extremely obscure' ..." I had assumed that the critic was saying Russell's writings lack clarity; and I ignored the other meaning, as in "Russell is an all-but-forgotten writer." But I don't think that's the fallacy we're supposed to spot. Russell goes on to say: "As I have a very intense desire to make my meaning plain ..." so I think, from the context, that the potential ambiguity is one which we can ignore. I'll stick with my analysis.
That's my point - I think Stace probably intended it in the sense of Russell being an unknown, unread writer, but Russell (intentionally) chose to take it as meaning that his work was unclear. Hence, Russell engaged in a bit of equivocation about the meaning of the word "obscure" as a flippant way of dismissing a critic that he didn't take all that seriously to begin with - if you're familiar with Stace, it's not at all surprising that Russell didn't take him particularly seriously.
And the ultimate irony is, of course, that nowadays, sixty-odd years later, W.T. Stace is the one who is obscure, albeit perhaps not "extremely" obscure ;)