Chapter one of "The Revolution Betrayed" is praising the Soviet rise in comparison to the Russian Empire, however chapter two moves on to Lenin's great question of "who shall prevail?" and thus it becomes a comparison, at it must be, of Capitalism vs Socialism. His point of the book is just that the USSR cannot possibly strive to surpass capitalism because it's methods are inadequate, the worker's lack the skill of the western workers, and so on due to the fact that the Imperial government didn't have the methods to begin with it, so Russia started with next to nothing. I guess to say, his point is that the USSR is rather hopeless while standing alone in the world since it must learn it's industrial means on it's own and shows economic and realistic data to back up this point. (He is making his beginning arguments for the permanent revolution and against the isolationism of Socialism in one State)
On the note of how he got the numbers, I believe the Soviet government made these relatively available to the world as a sort of "look how fast we are advancing" kind of thing so I wouldn't think it hard of him to find this information at all. Note how in chapter 2 he quotes Molotov, so this date was likely in Soviet media as well.