Most of the book is about Imperialism and the role of capitalist oligarchy, in fact Corbyn was quite specific about what aspect of the book he was calling "brilliant":
You can read his entire foreword here:
http://www.spokesmanbooks.com/Spokes.../130Corbyn.pdf
Yes the author was anti-Semitic, but that was unfortunately common at the time.
If you give a pass to fiction, then what about Henry Ford, Hugo Boss or Coco Chanel? Is anyone who quotes Ford, wears a Hugo Boss suit or Chanel earrings an anti-Semite?
I'm sorry, it's fair to criticise him for the foreword and ask him to condemn the anti-Semitic parts of the book, but you're massively oversimplifying.