Interesting. I recently read "A People's History of the World", and it also claimed that what happened last century in the Soviet Union was not true socialism; that author sometimes referred to the Soviet/Stalinist system as "state capitalism".
I haven't read anything that would cause me to assign the name "capitalism" to any aspect of the Soviet system. The phrase I hear most is "command economy." From my understanding, the key aspect of "capitalism" is the market and market forces as the moderating action over prices, scarcity, etc. Under Marxism/Leninism, the planned economy dictated prices, mandated scarcity and abundance, and assigned profitability. In theory, this was all accomplished through science in altruism. In practice it was accomplished through cronyism and opportunism. The czarist system was replaced with the nomenklatura, who were every bit as hereditary and protected a class as the bourgeoisie.
I thought it was odd usage also, but I believe the author chose that term to emphasise that the Soviet system was in no way run by or for the workers, in spite of the socialist rhetoric. Stalin wanted a productive economy, and he got one, at a huge human cost.