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 ;)
I donno ... That's a difficult interpretation for me. Russell, during his lifetime, was an international celebrity. The only thing that makes sense (to me) is that the critic was talking about the clarity of Russell's his writing -- and that too is nonsensical. Either way, it was neat for Russell, whose reputation for brilliance was unquestioned, to dismiss someone he regarded as a hack critic by saying: "Well, I guess you must be right." I'll still stick with my analysis, but I've been wrong before. As you know, in his declining years, Russell became a kook for pacifism, but in his prime he was awesome.