Whether or not Russell actually liked Lenin personally or agreed with his ideas he undoubtedly noted that those ideas had had, and would continue to have, great influence over world politics in the 20th century.
It is important to remember that the word 'great' as used by Russell did not necessarily imply personal admiration. It can be used dispassionately. Whatever his feelings about Lenin in 1920 or so, he disapproved of him in later years.
Have you read "China and Albert Einstein" by Danian Hu, where he says that Russell said those things in Japan?
... or "30 Years' Review of China's Science & Technology, 1949-1979" which says he said it in China?
Or "Uncertain Paths to Freedom: Russia and China, 1919-22" by Bertrand Russell, Richard A. Rempel, where I see this: